tools/Makefile: unset MAKELEVEL before building QEMU

tools/Makefile: unset MAKELEVEL before building QEMU

Since QEMU commits aef45d51d1204f3335fb99de6658e0c5612c2b67
"build: automatically handle GIT submodule checkout for dtc"
the QEMU makefiles rely on the variable MAKELEVEL to make a decision on
whether to update some git submodules or not. Since we call QEMU build
from within the Xen one, MAKELEVEL would already be greater than 0 and
the git submodules would not be updated and QEMU would fail to build.

Fix this by removing MAKELEVEL from the environment before trying to
build QEMU.

Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
Release-acked-by: Julien Grall <julien.grall@linaro.org>