plugins: splash: two-step: Secure boot check and a warning image

plugins: splash: two-step: Secure boot check and a warning image

Secure boot is used against several security threats when malware tries to
infect the firmware of the system. Users may inadvertently disable or
software may intentionally disable the secure boot. Consequently, the
system is running on an insecure platform with incorrect configuration. If
Plymouth could offer a warning to the user, the user could reboot and
reconfigure their system or asks for help immediately.

This work can be used to check the secure boot configuration and put a red
warning image on the screen if the secure boot is disabled. Also, this
check can be utterly disabled through the kernel parameter for testing.
If the parameter "secure_boot.warn_if_disabled=false" appears in the
kernel parameter, the secure boot check will be disabled.

Signed-off-by: Kate Hsuan <hpa@redhat.com>