doc: PORTING: drop flash_area_read_is_empty()

Update PORTING guide dropping `flash_area_read_is_empty`.

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

boot: Harden critical path against fault attacks

Add fault attack mitigation measures to code vital for the correct
validation of images.

Change-Id: If6eb1110a8c2966faf105d07ad2e95482a80a8d9
Signed-off-by: Raef Coles <raef.coles@arm.com>
Signed-off-by: Tamas Ban <tamas.ban@arm.com>

Release v1.6.0

Release v1.6.0-rc2

Release 1.6.0-rc1

Release v1.5.0