Bluetooth: Mesh: Improve logic for serving devices erase capabilities

The commit fixes issue where flash_area_flatten has been used where
code was only supposed to erase devices by hardware requirement prior
to write, by replacing the call with flash_area_erase and supporting
logic to select proper path.
There have been following Kconfig options added:
 - CONFIG_BT_MESH_BLOB_IO_FLASH_WITHOUT_ERASE
 - CONFIG_BT_MESH_BLOB_IO_FLASH_WITH_ERASE
that are available for user depending on devices in the system and allow
to turn off paths that are not used by BLOB IO; for example if user
never writes to device with erase CONFIG_BT_MESH_BLOB_IO_FLASH_WITH_ERASE
will disable the path.
Both Kconfig options are y by default and enable all paths.

Signed-off-by: Dominik Ermel <dominik.ermel@nordicsemi.no>