FROMLIST: binman: Support marking FMAP areas as preserved

Add an entry flag called 'preserve' to indicate that an entry should be
preserved by firmware updates. Propagate this to FMAP too.

BUG=b:268750177
TEST=binman test

Change-Id: Iebc71f0bbadc0604c0cbd777151bd8bb29cbffbc
Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/u-boot/+/4246314
Reviewed-by: Wai-Hong Tam <waihong@google.com>
Commit-Queue: Wai-Hong Tam <waihong@google.com>
(cherry picked from commit 07e9462cf3ce575d25300a11e0ac3013939f6ec3)
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/u-boot/+/4500973