mgmt: updatehub: Enable flash sha verification

Currently updatehub trust on MCUboot to test a new image. The process
is executed on next boot after a validate the downloaded image.  To
have a more reliable system, is recommended run the SHA-256 algorithm
to attest that firmware was properly stored on the flash memory.  This
implements the use of flash_img_check to achieve that, and as a
consequence, add a new level of trust that avoids an useless reboot
on the system.

Updatehub uses two SHA-256 verification.  One is used to check the
data stream at download.  The second one, realise the verification
reading from flash.  Users have the possibility to chose only one
option or keep both verifications.  However, it is adviced keep
always both verifications enabled to get consistent reports on
back end.

Signed-off-by: Gerson Fernando Budke <gerson.budke@ossystems.com.br>