Add FET swtich for battery ADC

Improve rn42_task time resolution

Update MEMO and remove a build option to save flash

Monitor battery and alert low voltage

Add battery management

Fix rn42_linked() to use pullup

Add files missing for JP support.

Add support for HHKB JP

Update MEMO.txt

Add connection indicator

Add initial setting of Linked pin

Add battery monitor and LED control

Add help message

Add rn42_lined(): connection status(GPIO2)

Add flow control by firmware to serial_uart

Clean up mbed_onekey

Mkdir rn42

Add rn42_task

Fix rn42.h API

Add software UART and battery monitor