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.

genimage.sh

@@ -14,14 +14,18 @@ cleanup() {
 
 . ./config
 
+O="${1}"
+IMAGESDIR="${2}"
+
 trap cleanup INT TERM QUIT EXIT
 tmproot=$(mktemp -d)
 tmppath=$(mktemp -d)
 
+cd "${O}"
 genimage \
     --rootpath "${tmproot}" \
     --tmppath "${tmppath}" \
-    --inputpath images/ \
-    --outputpath images/ \
+    --inputpath "${IMAGESDIR}" \
+    --outputpath "${IMAGESDIR}" \
     --mkdosfs mkfs.vfat \
-    --config genimage.cfg
+    --config "${OLDPWD}"/genimage.cfg