Merge changes from apertis/v2020-updates into apertis/v2020

Signed-off-by: Frédéric Danis <frederic.danis@collabora.com>

Current asserts on AumBootState object in aum_boot_state_*() functions
ends up when there is no boot state backend available, i.e. for UEFI AMD64
targets, by trying to rollback the OS.
Replacing those asserts by tests returning default values allows AUM to
handle this case gracefully and with better debug message.

Signed-off-by: Frédéric Danis <frederic.danis@collabora.com>