fix the build for standalone stm32h7 lib.

[slorquet]

Hi,

Here is a patch to fix the stm32h7 build, otherwise the final link just complains:

arm-none-eabi/bin/ld: bootloader.o: in function `_write':
arm-none-eabi/bin/ld: bootloader.c:217: undefined reference to `usart_send_blocking'
arm-none-eabi/bin/ld: bootloader.o: in function `usart_setup':
undefined reference to `usart_set_baudrate'
arm-none-eabi/bin/ld: bootloader.c:64: undefined reference to `usart_set_databits'
arm-none-eabi/bin/ld: bootloader.c:65: undefined reference to `usart_set_stopbits'
arm-none-eabi/bin/ld: bootloader.c:66: undefined reference to `usart_set_mode'
arm-none-eabi/bin/ld: bootloader.c:67: undefined reference to `usart_set_parity'
arm-none-eabi/bin/ld: bootloader.c:68: undefined reference to `usart_set_flow_control'
arm-none-eabi/bin/ld: bootloader.c:71: undefined reference to `usart_enable'

Thanks.

[karlp]

me@me.net? I don't ask for much, but somethign that can actually maybe reach people, or at least uniquely identify them please.