oeqa/selftest/wic: Add tests for kernel image installation

oeqa/selftest/wic: Add tests for kernel image installation

- test_skip_kernel_install: This test verifies that the kernel is not
installed in the boot partition when the
'install-kernel-into-boot-dir' parameter is set to false.
- test_kernel_install: This test verifies that the kernel is installed
in the boot partition when the 'install-kernel-into-boot-dir'
parameter is set to true.

Both tests use a WKS (Kickstart) file to specify the desired
configuration, build a disk image using WIC, and extract the disk
image to a temporary directory to verify the results.

Signed-off-by: Kareem Zarka <kareem.zarka@huawei.com>
Signed-off-by: Christian Taedcke <christian.taedcke@weidmueller.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>