test/py: Create a test for launching UEFI binaries from FIT images

This test verifies the implementation of the 'bootm' extension that
handles UEFI binaries inside FIT images (enabled via CONFIG_BOOTM_EFI).

Signed-off-by: Cristian Ciocaltea <cristian.ciocaltea@gmail.com>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>