fs_mgr_avb: fix return value check of fs_mgr_get_boot_config()

fs_mgr_get_boot_config() returns true/false but the return value check
in current fs_mgr_avb is for 0/1. This was introduced during a refactoring.

Check true/false for the return value.

Bug: 33254008
Test: manual test AVB on bullhead
Change-Id: I72c366627214df4a99c4d9cf1eb577e94f7afb31