meson: Make verify-full-signature stand-alone

The new verify-full-signature config option requires that the legacy
config option verify-signature to be also enabled or it's a no-op.
Having to specify two different options to enable the full signature
verification is not user friendly.

Allow the verify-full-signature to turn on signature checking the same
as the verify-signature option, and add a comment that the new
verify-full-signature is the preferred option instead of the legacy
verify-signature.

Tested: Verified that compiling with -Dverify-full-signature=enabled
        defined WANT_SIGNATURE_VERIFY and WANT_SIGNATURE_FULL_VERIFY.
        Compiling with -Dverify-signature=enabled only enabled
        WANT_SIGNATURE_VERIFY. And either option compiled the
        image_verify.cpp file.

Change-Id: Ib3a674a51d7a3d4a122a6ceb35f1695831314867
Signed-off-by: Adriana Kobylak <anoo@us.ibm.com>
diff --git a/meson_options.txt b/meson_options.txt
index 8b40d0e..355773c 100644
--- a/meson_options.txt
+++ b/meson_options.txt
@@ -20,7 +20,7 @@
 option('oe-sdk', type: 'feature', description: 'Enable OE SDK')
 
 option('verify-signature', type: 'feature',
-    description: 'Enable image signature validation.')
+    description: 'LEGACY: Use verify-full-signature instead. Enable image signature validation.')
 
 option('verify-full-signature', type: 'feature',
     description: 'Enable image full signature validation.')