efi_selftest: check installation of the device tree

efi_selftest: check installation of the device tree

The unit test checks if a device tree is installed. It requires that the
'compatible' property of the root node exists. If available it prints the
'serial-number' property.

The serial-number property is derived from the environment variable
'serial#'. This can be used to check if the image_setup_libfdt() function
is executed.

A Python test is supplied. It sets a value for serial# and checks that the
selftest shows this as serial-number.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Alexander Graf <agraf@suse.de>