qemu: correct kernel headers check when major numbers differ

commit f7add51c (qemu: add host/target Linux version check) added a
version check between the host kernel version and the version of kernel
headers used by the toolchain, but the logic would fail unless BOTH major
and minor versions were >=, which isn't true for E.G. host kernel = 3.2 and
toolchain 2.6.x.

Instead calculate a single version number (as major << 8 + minor) and
compare that.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>