common: add support to sign boot image

adding rules for generating signed boot image

Change-Id: I53710c2b4b8e8750c29d9c46c51a3e4ff0f270d2
Sign recovery image

LK now requires recovery image to be signed. This
is a new change to ensure that only signed images
are used.

Change-Id: I25fef9c311370e3f26d2d09b577575cd2a16f880
Merge "common: add support to sign boot image"
Merge "Sign recovery image"