Updates to Zero bootloader configuration

- Use pin 7 of Arduino Zero to force bootloader start
- Use RX/TX pin 0/1 of Arduino Zero to communicate in UART mode

bootloaders/zero/main.h

@@ -34,7 +34,8 @@
 #define CPU_FREQUENCY                     8000000
 
 #define APP_START_ADDRESS                 0x00002000
-#define BOOT_LOAD_PIN                     PIN_PA15
+#define BOOT_LOAD_PIN                     PIN_PA21 //Pin 7
+//#define BOOT_LOAD_PIN                     PIN_PA15 //Pin 5
 #define BOOT_PIN_MASK                     (1U << (BOOT_LOAD_PIN & 0x1f))
 #define FLASH_WAIT_STATES                 1