Allow more than one '=' in avb_extra_custom_image_extra_args.

avb_extra_custom_image_extra_args can be set to
'oem=--signing_helper_with_files=/tmp/avbsigner.sh' on signing server.
The second '=' shouldn't be splitted in this case.

BUG: 154171021
Test: Manually sign target zip files which has custom images.
  sign_target_files_apks -d certs --avb_extra_custom_image_key \
    oem=oem_rsa4096.pem --avb_extra_custom_image_algorithm \
    oem=SHA256_RSA4096 --avb_extra_custom_image_extra_args \
    oem="--signing_helper_with_files=/tmp/avbsigner.sh" \
    xxxx-target_files.zip signed.zip

Change-Id: I815d574f791734d4c6a25a9f8d827b5b4f93d7cc