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.

post-build.sh

@@ -5,9 +5,9 @@ set -e
 
 . ./config
 
-cp -auv \
+cp -uv \
     /usr/${target}/boot/*.bin \
     /usr/${target}/boot/*.dat \
     /usr/${target}/boot/*.elf \
     config.txt \
-    output/efi-part/
+    "$1"/efi-part/