selftest: uboot: remove duplicated KVM presence test

selftest: uboot: remove duplicated KVM presence test

test_boot_uboot_kvm_to_full_target test, needs a working KVM.
It checked it with the kvm-ok tool but that tool is not documented as a
requirement. Also, the check is also done by runqemu called later in the
test.

Remove it from the test and let runqemu have the responsibility of
checking for KVM.

Note that the KVM check now happens *after* the image build instead of
before it.

Signed-off-by: Yoann Congal <yoann.congal@smile.fr>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>