default.avbkey: fix 0 key size
Previous commit which introduces avbkey just create a empty file, so AVB not working.
Commit default key use in Android build system to support AVB feature.
Fixes: 7988f626 ("common: avb_verify: Add CONFIG_AVB_PUBKEY_FILE to specify the root avb public key") Signed-off-by: Guillaume La Roque glaroque@baylibre.com