Adding support for 1Bitsy

So I have a PR open against mbed which enables the MCU_STM32F415xG target. In the mean time I’ve been working off that branch on the 1bitsy support. Code is here: James Harton / mbed-os-1bitsy · GitLab

The current status is that the code compiles and runs, but ThisThread::sleep_for never returns. When I inspect it with the debugger I can step through it fine, however. My assumption is that I haven’t set up the system clock source or a timer correctly (I’m basically just working straight from the MCU_STM32F415xG target with very little added code for now). Any pointers would be appreciated.

Debug output is here: `ThisThread::sleep_for` not returning. ($2005007) · Snippets · James Harton / mbed-os-1bitsy · GitLab