boot: bootutil: drop flash_area_read_is_empty

Removes the current `flash_area_read_is_empty` which lacked a bit of
clarity in its naming and error handling, as well as requiring an
extra API in the flash map, and switches to using an internal function
`bootutil_buffer_is_erased`.

Code that was previously using `flash_area_read_is_empty` must now be
updated to do a `flash_area_read` followed by a call to
`bootutil_buffer_is_erased` with the read buffer.

The proposal was previously discussed here:

https://github.com/zephyrproject-rtos/zephyr/pull/28519



Signed-off-by: Fabio Utzig <fabio.utzig@nordicsemi.no>