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-update.sh

@@ -5,11 +5,13 @@ set -e
 
 . ./config
 
-cd images
+IMAGESDIR="$1"
+
+cd "${IMAGESDIR}"
 sha256sum firmware.img > digests
 sha256sum rootfs.squashfs >> digests
-ln ../install-update.sh install
-tar -c --zstd -f ../output/update.tar.zstd \
+cp -u "${OLDPWD}"/install-update.sh install
+tar -c --zstd -f update.tar.zstd \
     digests \
     firmware.img \
     rootfs.squashfs \