lib: crypto: add pkcs7_digest()

This function was nullified when the file, pkcs7_verify.c, was imported
because it calls further linux-specific interfaces inside, hence that
could lead to more files being imported from linux.

We need this function in pkcs7_verify_one() and so simply re-implement it
here instead of re-using the code.

Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>