Added bit mask and constant for SerialConfig parameter

Permit use bit config avoiding test/compare all "hard-coded" values.
example
```arduino
if (section & CFG_HLP_MODBUS) {
DebuglnF("\r\n===== Modbus");
DebugF("Speed       : "); Debug(config.modbus.baud);

switch (config.modbus.proto & UART_NB_BIT_MASK) {
case UART_NB_BIT_5: Debug(" 5"); break;
case UART_NB_BIT_6: Debug(" 6"); break;
case UART_NB_BIT_7: Debug(" 7"); break;
case UART_NB_BIT_8: Debug(" 8"); break;
default : Debug(" ?"); break;
}
switch (config.modbus.proto & UART_PARITY_MASK) {
case UART_PARITY_NONE: Debug("N"); break;
case UART_PARITY_EVEN: Debug("E"); break;
case UART_PARITY_ODD : Debug("O"); break;
default : Debug("?"); break;
}
switch (config.modbus.proto & UART_NB_STOP_BIT_MASK) {
case UART_NB_STOP_BIT_0: Debug("0"); break;
case UART_NB_STOP_BIT_1: Debug("1"); break;
case UART_NB_STOP_BIT_15: Debug("15"); break;
case UART_NB_STOP_BIT_2 : Debug("2"); break;
default : Debug("?"); break;
}

DebugF("\r\nSlave Addr  : "); Debugln(config.modbus.slave);
DebugF("Holding Reg : 0x"); Debugln2(config.modbus.hr_addr,HEX);
DebugF("Enable Pin  : "); Debugln(config.modbus.en_pin);
}
```

Author: hallard

cores/esp8266/uart.h

@@ -34,6 +34,49 @@ extern "C" {
 #define UART_NO -1
 
 // Options for `config` argument of uart_init
+#define UART_NB_BIT_MASK      0B00001100
+#define UART_NB_BIT_5         0B00000000
+#define UART_NB_BIT_6         0B00000100
+#define UART_NB_BIT_7         0B00001000
+#define UART_NB_BIT_8         0B00001100
+
+#define UART_PARITY_MASK      0B00000011
+#define UART_PARITY_NONE      0B00000000
+#define UART_PARITY_EVEN      0B00000010
+#define UART_PARITY_ODD       0B00000011
+
+#define UART_NB_STOP_BIT_MASK 0B00110000
+#define UART_NB_STOP_BIT_0    0B00000000
+#define UART_NB_STOP_BIT_1    0B00010000
+#define UART_NB_STOP_BIT_15   0B00100000
+#define UART_NB_STOP_BIT_2    0B00110000
+
+#define UART_5N1 ( UART_NB_BIT_5 | UART_PARITY_NONE | UART_NB_STOP_BIT_1 )
+#define UART_6N1 ( UART_NB_BIT_6 | UART_PARITY_NONE | UART_NB_STOP_BIT_1 )
+#define UART_7N1 ( UART_NB_BIT_7 | UART_PARITY_NONE | UART_NB_STOP_BIT_1 )
+#define UART_8N1 ( UART_NB_BIT_8 | UART_PARITY_NONE | UART_NB_STOP_BIT_1 )
+#define UART_5N2 ( UART_NB_BIT_5 | UART_PARITY_NONE | UART_NB_STOP_BIT_2 )
+#define UART_6N2 ( UART_NB_BIT_6 | UART_PARITY_NONE | UART_NB_STOP_BIT_2 )
+#define UART_7N2 ( UART_NB_BIT_7 | UART_PARITY_NONE | UART_NB_STOP_BIT_2 )
+#define UART_8N2 ( UART_NB_BIT_8 | UART_PARITY_NONE | UART_NB_STOP_BIT_2 )
+#define UART_5E1 ( UART_NB_BIT_5 | UART_PARITY_EVEN | UART_NB_STOP_BIT_1 )
+#define UART_6E1 ( UART_NB_BIT_6 | UART_PARITY_EVEN | UART_NB_STOP_BIT_1 )
+#define UART_7E1 ( UART_NB_BIT_7 | UART_PARITY_EVEN | UART_NB_STOP_BIT_1 )
+#define UART_8E1 ( UART_NB_BIT_8 | UART_PARITY_EVEN | UART_NB_STOP_BIT_1 )
+#define UART_5E2 ( UART_NB_BIT_5 | UART_PARITY_EVEN | UART_NB_STOP_BIT_2 )
+#define UART_6E2 ( UART_NB_BIT_6 | UART_PARITY_EVEN | UART_NB_STOP_BIT_2 )
+#define UART_7E2 ( UART_NB_BIT_7 | UART_PARITY_EVEN | UART_NB_STOP_BIT_2 )
+#define UART_8E2 ( UART_NB_BIT_8 | UART_PARITY_EVEN | UART_NB_STOP_BIT_2 )
+#define UART_5O1 ( UART_NB_BIT_5 | UART_PARITY_ODD  | UART_NB_STOP_BIT_1 )
+#define UART_6O1 ( UART_NB_BIT_6 | UART_PARITY_ODD  | UART_NB_STOP_BIT_1 )
+#define UART_7O1 ( UART_NB_BIT_7 | UART_PARITY_ODD  | UART_NB_STOP_BIT_1 )
+#define UART_8O1 ( UART_NB_BIT_8 | UART_PARITY_ODD  | UART_NB_STOP_BIT_1 )
+#define UART_5O2 ( UART_NB_BIT_5 | UART_PARITY_ODD  | UART_NB_STOP_BIT_2 )
+#define UART_6O2 ( UART_NB_BIT_6 | UART_PARITY_ODD  | UART_NB_STOP_BIT_2 )
+#define UART_7O2 ( UART_NB_BIT_7 | UART_PARITY_ODD  | UART_NB_STOP_BIT_2 )
+#define UART_8O2 ( UART_NB_BIT_8 | UART_PARITY_ODD  | UART_NB_STOP_BIT_2 )
+
+/*
 #define UART_5N1 0x10
 #define UART_6N1 0x14
 #define UART_7N1 0x18
@@ -58,6 +101,7 @@ extern "C" {
 #define UART_6O2 0x37
 #define UART_7O2 0x3b
 #define UART_8O2 0x3f
+*/
 
 // Options for `mode` argument of uart_init
 #define UART_FULL     0