selftest/devtool: Modify test to use default config

selftest/devtool: Modify test to use default config

Modify “test_devtool_virtual_kernel_modify” to be executed with default
configuration defined on oe-selftest test suite. A shorter string was
added to the the header file to avoid overlapped sections when building
kernel for qemux86-64.

[Yocto #11300]

Signed-off-by: Jose Perez Carranza <jose.perez.carranza@linux.intel.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>