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-uboot.sh

@@ -5,12 +5,14 @@ set -e
 
 . ./config
 
+O="${1}"
+
 ./patch-uboot.sh
 cd u-boot
 cat configs/rpi_4_defconfig ../u-boot.config > configs/yellow_defconfig
-make yellow_defconfig
-CROSS_COMPILE=${target}- make
+make O="${O}"/u-boot yellow_defconfig
+CROSS_COMPILE=${target}- make O="${O}"/u-boot -j$(nproc)
 cd ..
 
-mkdir -p output/efi-part
-cp -au u-boot/u-boot.bin output/efi-part
+mkdir -p "${O}"/efi-part
+cp -u "${O}"/u-boot/u-boot.bin "${O}"/efi-part