init: Wait for /dev/hvc1 during ARCVM first-stage mount

This commit introduces a function to allow ARC-specific logic during
first-stage mount by checking for the existence of an indicator file
at the path "/is_arcvm".

ARC uses the virtio-console device `hvc1` to pass byte data to Android
before second-stage init. Ensure that /dev/hvc1 is initialized before
ARCVM continues booting, but allow other devices to bypass this device
initialization.

Bug: 325538592
Test: boot ARC, ensure /dev/hvc1 can be read during PropertyInit
Change-Id: Ic258b7b004b59da462f4990131a5c11fc94eca62