lib: rsa: function to verify a signature against a hash

rsa_verify() expects a memory region and wants to do the hashing itself,
but there may be cases where the hashing is done via other means,
like hashing a squashfs rootfs.

So add rsa_verify_hash() to allow verifiying a signature against
an existing hash. As this entails the same verification routines
we can just move the relevant code over from rsa_verify() and also
call rsa_verify_hash() from there.

Signed-off-by: Heiko Stuebner <heiko.stuebner@theobroma-systems.com>