lpc176x: Move bootloader_request() from usbserial.c to main.c

Move bootloader_request() function so that it can be used when not
using USB.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>

src/lpc176x/internal.h

@@ -23,5 +23,6 @@ int is_enabled_pclock(uint32_t pclk);
 void enable_pclock(uint32_t pclk);
 uint32_t get_pclock_frequency(uint32_t pclk);
 void gpio_peripheral(uint32_t gpio, int func, int pullup);
+void usb_disconnect(void);
 
 #endif // internal.h