oeqa/selftest/imagefeatures: adding fitImage initramfs bundle testcase

oeqa/selftest/imagefeatures: adding fitImage initramfs bundle testcase

This commit provides a testcase for the initramfs bundle support implemented
in kernel-fitimage.bbclass

The testcase verifies the content of the initramfs bundle node in the FIT
Image Tree Source (its).

The testcase is self-contained and the configurations are set by the test case itself.

To verify the initramfs bundle support, the testcase uses beaglebone-yocto machine.

This testcase can be run through the following command:

oe-selftest -r fitimage.FitImageTests.test_initramfs_bundle

Change-Id: I8ab8abf2c150ea515fd439784eb20c6b092bfbc5
Signed-off-by: Abdellatif El Khlifi <abdellatif.elkhlifi@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>