Make it possible to switch gcrypt/openssl via knob

Make it possible to switch gcrypt/openssl via knob

By building with "make USE_OPENSSL=1", OpenSSL will be used in
libcrypt's stead. This also allows for properly depend just on the
keys-*.c source file that is actually going to be used.

Note that the all target is moved up so that it still hits as default
target.

Signed-off-by: Diego E. 'Flameeyes' Pettenò <flameeyes@gmail.com>
Signed-off-by: Luis R. Rodriguez <lrodriguez@atheros.com>