Kconfig: FIT_SIGNATURE should not select RSA_VERIFY

FIT signatures can now be implemented with ECDSA. The assumption that
all FIT images are signed with RSA is no longer valid. Thus, instead
of 'select'ing RSA, only 'imply' it. This doesn't change the defaults,
but allows one to explicitly disable RSA support.

Signed-off-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Igor Opaniuk <igor.opaniuk@foundries.io>
Reviewed-by: Patrick Delaunay <patrick.delaunay@foss.st.com>