Add update_bootloader_message() to fix two-step OTAs.

This is a retry of commit 7e31f421.

Commit bd56f159 switches to calling
write_bootloader_message(<options>) in get_args(), which
unintentionally resets the stage field thus breaks two-step OTAs.

This CL adds update_bootloader_message(<options>), which only sets
the command field (to "boot-recovery") and the recovery field (with
the specified options).

Bug: 33534933
Test: Apply a two-step package.
Test: recovery_component_test passes.
Change-Id: Ie0b1ed4053d2d3c97d9cb84310d616b28fcfc72e