bootutil/crypto: Fix local variable name typo for PSA Crypto key parse

Fix a local variable name typo in parse public key function
for the PSA Crypto abstraction, and at the same time put the
memcmp under ifdefs.

Signed-off-by: Antonio de Angelis <Antonio.deAngelis@arm.com>
Change-Id: Icadca37e4207ad703a853ea720a053aa2ba76411