Add kcmdlinectrl for setting the kcmdline bootloader message

This utility makes it possible to read and write to the kcmdline
bootloader message. This message can be used to tell the bootloader to
set kcmdline flags for enabling features in the kernel.

Test: Verified that a custom bootloader is able to read the data
Bug: 278052745
Change-Id: I5f13a9bdff940517cb7b880815dfb8f396fc3844
Signed-off-by: Alice Ryhl <aliceryhl@google.com>
4 files changed