Allow vendor_init to write to misc_block_device

Vendors may use this to write custom messages to their bootloader, and
as the bootloader is under vendor control, this makes sense to allow.

Bug: 77881566
Test: build
Merged-In: I78f80400e5f386cad1327a9209ee1afc8e334e56
Change-Id: I78f80400e5f386cad1327a9209ee1afc8e334e56
(cherry picked from commit db465285)