Add kcmdline bootloader message

Define a new struct in the misc partition for telling the bootloader to
set kcmdline flags for enabling experimental dogfood 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>