image: rsa: Move verification algorithm to a linker list

Move the RSA verification crytpo_algo structure out of the
crypto_algos array, and into a linker list.

Although it appears we are adding an #ifdef to rsa-verify.c, the gains
outweigh this small inconvenience. This is because rsa_verify() is
defined differently based on #ifdefs. This change allows us to have
a single definition of rsa_verify().

Signed-off-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>