DO NOT MERGE ANYWHERE Add comments and code about ro.boot.slot back

Taimen bootloader passes androidboot.slot for now.
This reverts commit 211a4a5d.

Bug: 68710251
Test: taimen successfully mounts partition in first stage
Change-Id: I046afc99120e7ec9ef4b8ea7a852eb70e7231ea9
Signed-off-by: Sandeep Patil <sspatil@google.com>