fastbootd: allow to fetch vendor_boot partition.

vendor_boot is labeled as boot_block_device. With
fastboot fetch command, fastbootd needs to read
the vendor_boot device and return it to the host.

Test: pass
Bug: 173654501
Change-Id: I197e39c9e7572dc9a714f36637c02ee9ead2e5f3
1 file changed