sysroot: Reword comment and use gboolean over bool, error handling

sysroot: Reword comment and use gboolean over bool, error handling

Be more explicit in the comment, and use gboolean over bool. Less header
inclusions when we use gboolean. Although bool is used in some places.
Write a separate _ostree_sysroot_parse_bootlink_aboot function for
aboot. Make is_aboot optional. Handle invalid androidboot karg and no
ostree and androidboot kargs differently.

Co-authored-by: Jonathan Lebon <jonathan@jlebon.com>
Signed-off-by: Eric Curtin <ecurtin@redhat.com>