build-kernel: Set USE=symlink as env var

Since this is the last vestige of the "host" Portage configuration
directory, we can remove it entirely by setting `USE=symlink` as an
environment variable.  This should be safe, since there are not many
other packages besides kernel sources that use this USE flag, and
they're not likely to be pulled in with this `emerge` invocation.

build-kernel.sh

@@ -7,8 +7,8 @@ O="${1}"
 
 . "${CONFIGDIR:=${PWD}}"/config
 
-PORTAGE_CONFIGROOT="${CONFIGDIR}"/portage/host \
-emerge -vnj ${kernel_pkg}
+USE=symlink \
+emerge -vnuUj ${kernel_pkg}
 
 export ARCH=arm64 CROSS_COMPILE=${target}-
 unset MAKEFLAGS MAKEOVERRIDES MAKELEVEL