oeqa/selftest: generalise test_devtool_virtual_kernel_modify

oeqa/selftest: generalise test_devtool_virtual_kernel_modify

Generalise this test so that it works on more than qemux86-64:

- Don't edit a file in arch/x86 to cause a rebuild, instead use init/
- Look for the edits in the build tree, as the deployed kernel could be
  of any type (zImage/bzImage/etc) and edits may be in the compressed
  part.

Also remove redundant checks on the result of runCmd(), as this will
raise AssertionError exceptions itself so the explicit asserts will
never trigger.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>