FastbootCmds: oem device-info: check if user public key is set

It is possible to flash to a virtual partition `avb_custom_key` to
set a custom AVB key.

1) generate a custom avb private key

openssl genpkey -algorithm RSA \
  -pkeyopt rsa_keygen_bits:4096 \
  -outform PEM \
  -out key_rsa4096.pem

2) Extract public key usingprivate key
  - avbtool extract_public_key --key key_rsa4096.pem --output key_rsa4096.pub

The resulting `key_rsa4096.pub` can be flashed using fastboot:
  - fastboot flash avb_custom_key key_ras4096.pub

Afterwards verify that the custom AVB key is set using:
  - fastboot oem device-info

You can also use the official AVB repo as reference:
  - https://android.googlesource.com/platform/external/avb/+/master/README.md#pixel-2-and-later

Change-Id: I6f65c58dd7f7cbdacfa02464d94815e5e3e6918f
Signed-off-by: Alexander Martinz <amartinz@shiftphones.com>
3 files changed