update makefiles
replace calls to standard memcmp with constant memcmp where necessary
make pkcs#1 decode functions constant-time

as proposed in RFC 3447 only one error return code is used when there are
errors while decoding the pkcs#1 format.
also, all steps are executed and only the "output" is skipped if something
went wrong.

Sorry this could break backwards compatibility, since there's no more
BUFFER_OVERFLOW messaging.
Former error-handling code could also be affected because now there's only
OK as return code in cases where "res" is also set to '1'.
adapt rsa_test() to modified pkcs#1 decoding routines