Kconfig: Update default hex values to start with 0x

Kconfig hex values don't need to be in quotes, and should start with
'0x'.  If the default value isn't set this way, Kconfig will add the
0x to the start, and the entry can be added unnecessarily to the
defconfig since it's "different" than what was set by the default.

A check for this has been added to the Kconfig lint tool.

Change-Id: I86f37340682771700011b6285e4b4af41b7e9968
Signed-off-by: Martin Roth <martinroth@google.com>
Reviewed-on: https://review.coreboot.org/16834
Tested-by: build bot (Jenkins)
Reviewed-by: Kyösti Mälkki <kyosti.malkki@gmail.com>

src/vendorcode/google/chromeos/Kconfig

@@ -212,7 +212,7 @@ config VBOOT_KEYBLOCK_VERSION
 
 config VBOOT_KEYBLOCK_PREAMBLE_FLAGS
 	hex "Keyblock preamble flags"
-	default 0
+	default 0x0
 
 endmenu # Keys