Support external build directory

When running inside a QEMU microvm with the source directory shared
via 9pfs, the kernel build process fails

> Error: Could not mmap file: vmlinux

Thus, we need to run the build in a path on a local filesystem.  To
support this, the Makefile now supports an `O` option, and all the build
scripts have been adjusted to make use of it as needed.

Since building in a local filesystem would ultimately discard the final
artifacts when the VM terminates, we need yet a different location for
the files we want to keep.  The `IMAGESDIR` option can be used to
specify this path.  This path can be on a shared filesystem, thus
saving the artifacts outside the microvm.

build-squashfs.sh

@@ -5,10 +5,12 @@ set -e
 
 . ./config
 
-mkdir -p images
+IMAGESDIR="${1}"
+
+mkdir -p "${IMAGESDIR}"
 mksquashfs \
     /mnt/gentoo \
-    images/rootfs.squashfs \
+    "${IMAGESDIR}"/rootfs.squashfs \
     -comp gzip \
     -ef squashfs.exclude \
     -no-exports \