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all FIT images are signed with RSA is no longer valid.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: NAlexandru Gagniuc <mr.nuke.me@gmail.com>
Reviewed-by: NSimon Glass <sjg@chromium.org>
Reviewed-by: NIgor Opaniuk <igor.opaniuk@foundries.io>
Reviewed-by: NPatrick Delaunay <patrick.delaunay@foss.st.com>

common/Kconfig.boot

@@ -76,8 +76,8 @@ config FIT_SIGNATURE
 	bool "Enable signature verification of FIT uImages"
 	depends on DM
 	select HASH
-	select RSA
-	select RSA_VERIFY
+	imply RSA
+	imply RSA_VERIFY
 	select IMAGE_SIGN_INFO
 	select FIT_FULL_CHECK
 	help
@@ -186,8 +186,8 @@ config SPL_FIT_SIGNATURE
 	select SPL_FIT
 	select SPL_CRYPTO
 	select SPL_HASH_SUPPORT
-	select SPL_RSA
-	select SPL_RSA_VERIFY
+	imply SPL_RSA
+	imply SPL_RSA_VERIFY
 	select SPL_IMAGE_SIGN_INFO
 	select SPL_FIT_FULL_CHECK