This commit makes the low-level flash C functions usable by code other than
flashbdev.c (eg by mboot). Changes in this commit are:
- flash_erase() and flash_write() now return an errno error code, a
negative value on error.
- flash_erase() now automatically locks the flash, as well as unlocking it.
- flash_write() now automatically unlocks the flash, as well as locking it.
- flashbdev.c is modified for the above changes.
Signed-off-by: Damien George <damien@micropython.org>
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