security/vboot: Add function to read UDC enable flag

This change adds a helper function to read USB Device Controller (UDC)
enable flag from VBNV.

BUG=b:78577893
BRANCH=poppy

Change-Id: Ifd1e9b0781ffee242d695b72287632bc944a50c7
Signed-off-by: Duncan Laurie <dlaurie@google.com>
Signed-off-by: Furquan Shaikh <furquan@google.com>
Reviewed-on: https://review.coreboot.org/25846
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Aaron Durbin <adurbin@chromium.org>

src/security/vboot/vbnv_layout.h

@@ -40,6 +40,7 @@
 #define DEV_FLAGS_OFFSET             4
 #define DEV_BOOT_USB_MASK               0x01
 #define DEV_BOOT_SIGNED_ONLY_MASK       0x02
+#define DEV_ENABLE_UDC                  0x40
 
 #define MISC_FLAGS_OFFSET            8
 #define MISC_FLAGS_BATTERY_CUTOFF_MASK  0x08