bootutil: (refactor) split image_validate.c into separate files

Move bootutil_find_key(), bootutil_img_hash(), and
bootutil_img_security_cnt() into their own source files for cleaner

No functional changes have been made.

Signed-off-by: Maulik Patel <maulik.patel@arm.com>
Change-Id: I5d3f4a1327356238e5865dba91b4ec251ffcf1aa