rsa: crash in br_i32_decode() called from rsa_gen_key_prop()

Fixes problem for unaligned 32bit big-endian access in
lib/rsa/rsa-keyprop.c.

Exchanges br_i32_decode() with get_unaligned_be32().

This will keep the unaligned access for architectures capable and will do
some byte-shift magic for the not so capable ones.

Reported-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-by: Robert Reither <robert.reither@external.thalesgroup.com>
Remove unused include.
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>