Add read_bootloader_message_from() and write_bootloader_message_to() to
allow specifying the BCB device (/misc).
Also add testcases for set_stage() and get_stage().
Test: recovery_component_test passes.
Test: Build a recovery image and apply a two-step OTA package.
Change-Id: If5ab06a1aaaea168d2a9e5dd63c07c0a3190e4ae
bootloader_messages merges bootloader_message_writer
and bootloader.cpp, so we can use the same library to
manage bootloader_message in normal boot and recovery mode.
Bug: 29582118
Change-Id: I9efdf776ef8f02b53911ff43a518e035e0c29618
(cherry picked from commit 2f272c0551)
