Support for conditionally mounting /vendor partition in Microdroid

first_stage_init will only mount the /vendor partition in Microdroid if
the androidboot.microdroid.mount_vendor=1 is provided in the kernel
cmdline.

Bug: 285855433
Test: atest MicrodroidTestApp
Change-Id: I5b840b5474bc52ec2696a0ba6ead0476acddfb1a