init: disable code touching bootloader_message on microdroid

Saves 11,315 bytes.

size -A out/soong/.intermediates/system/core/init/init_second_stage.microdroid/android_arm64_armv9-2a_cortex-a55/init

Bug: 416570070
Test: m
Flag: EXEMPT .
Change-Id: Ibe7455922b8b9c99d3d4ac7abc076eee27683524