Qmk_firmware: hhkb bluetooth support can not compile

[uuhan]

I clone the source and run make hhkb HHKB_RN42_ENABLE=yes.
avr-gcc version 8.1.0
It failed to compile with the following error:

keyboards/hhkb/rn42/rn42.c: In function 'rn42_init':
keyboards/hhkb/rn42/rn42.c:48:5: error: implicit declaration of function 'serial_init' [-Werror=implicit-function-declaration]
serial_init();
^
keyboards/hhkb/rn42/rn42.c: In function 'rn42_getc':
keyboards/hhkb/rn42/rn42.c:53:5: error: implicit declaration of function 'serial_recv2' [-Werror=implicit-function-declaration]
return serial_recv2();
^
keyboards/hhkb/rn42/rn42.c: In function 'rn42_putc':
keyboards/hhkb/rn42/rn42.c:75:5: error: implicit declaration of function 'serial_send' [-Werror=implicit-function-declaration]
serial_send(c);

[phongnh]

It seems that serial_uart.c is not included properly when building HHKB firmware. I tried to copy both serial.h and serial_uart.c from qmk_firmware/tmk_core/protocol to qmk_firmware/keyboards/hhkb and building HHKB firmware worked fine.

[uuhan]

@phongnh Hi, It works fine! Thank you. I think you can make a pr to qmk :)