chore(api-lock): Port to new mutex API

Using a bare FreeRTOS mutex is deprecated.  Replace it with the new
`struct mutex`.  This should increase stability.

Additionally, fix a bug in `lifecycle.c:do_load()` where the function
would return without unlocking the API mutex if `hardware_reset()`
returns an error.

Signed-off-by: Rahix <rahix@rahix.de>