fs_mgr: add a generic fs_mgr_get_boot_config internal API

depending on when fs_mgr is trying to read the configuration passed into
the kernel commandline, it may be able to read it successfully.
Specially in the case when init has not initialized properties.

This change adds a new fs_mgr_get_boot_config() API to be used by all
fs_mgr code in order to get filesystem parameters specified in kernel
command line or device tree. This way the fs_mgr code doesn't have to
handle the "early" cases separately anywhere.

Test:
Tested angler boot with both /system and /vendor mounted in init
first stage.
Tested sailfish to make sure /vendor can be continued to be
mounted early without verity

Change-Id: I9a44cdfc32681f714c5d73ae55c3deda95c02545