updater: Refactor set_stage() and get_stage() functions.

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