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.build b/meson.build
index f06090e..0a7a6a6 100644
--- a/meson.build
+++ b/meson.build
@@ -53,7 +53,9 @@
 
 # Configurable features
 conf.set('HOST_BIOS_UPGRADE', get_option('host-bios-upgrade').enabled())
-conf.set('WANT_SIGNATURE_VERIFY', get_option('verify-signature').enabled())
+conf.set('WANT_SIGNATURE_VERIFY', \
+    get_option('verify-signature').enabled() or \
+    get_option('verify-full-signature').enabled())
 conf.set('WANT_SIGNATURE_FULL_VERIFY', get_option('verify-full-signature').enabled())
 
 # Configurable variables
@@ -171,7 +173,8 @@
     )
 endif
 
-if get_option('verify-signature').enabled()
+if (get_option('verify-signature').enabled() or \
+    get_option('verify-full-signature').enabled())
     image_updater_sources += files(
         'utils.cpp',
         'image_verify.cpp',