FIT: Modify option FIT_SIGNATURE in Kconfig

For FIT signature based approach to work, RSA library needs to be selected.
The FIT_SIGNATURE option in Kconfig is modified to automatically select RSA.
Selecting RSA compiles the RSA library required for image verification.

Signed-off-by: Ruchika Gupta <ruchika.gupta@freescale.com>
CC: Simon Glass <sjg@chromium.org>
Acked-by: Simon Glass <sjg@chromium.org>