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
diff --git a/updater/Android.mk b/updater/Android.mk
index 3c1d0d4..5d328a3 100644
--- a/updater/Android.mk
+++ b/updater/Android.mk
@@ -27,12 +27,14 @@
     libedify \
     libziparchive \
     libotautil \
+    libbootloader_message \
     libutils \
     libmounts \
     libotafault \
     libext4_utils_static \
     libfec \
     libfec_rs \
+    libfs_mgr \
     liblog \
     libselinux \
     libsparse_static \