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-kernel.sh

@@ -3,16 +3,20 @@
 
 set -e
 
+O="${1}"
+
 . ./config
 
 emerge -vnj ${kernel_pkg}
 
 export ARCH=arm64 CROSS_COMPILE=${target}-
-mkdir -p linux
-cd linux
+unset MAKEFLAGS MAKEOVERRIDES MAKELEVEL
+mkdir -p "${O}"/linux
 /usr/src/linux/scripts/kconfig/merge_config.sh -m \
+    -O "${O}"/linux \
     /usr/src/linux/arch/*/configs/${kernel_defconfig}_defconfig \
-    ../linux.config
+    linux.config
+cd "${O}"/linux
 make -C /usr/src/linux O=${PWD} olddefconfig
 make -j$(nproc)
 touch arch/arm64/boot/Image.gz
@@ -22,14 +26,14 @@ cd -
 
 printf 'Installing Kernel %s ...\n' "${kver}"
 mkdir -p /mnt/gentoo/boot
-cp -au linux/arch/arm64/boot/Image.gz /mnt/gentoo/boot/vmlinuz-${kver}
-cp -au linux/.config /mnt/gentoo/boot/config-${kver}
-cp -au linux/System.map /mnt/gentoo/boot/System.map-${kver}
+cp -au "${O}"/linux/arch/arm64/boot/Image.gz /mnt/gentoo/boot/vmlinuz-${kver}
+cp -au "${O}"/linux/.config /mnt/gentoo/boot/config-${kver}
+cp -au "${O}"/linux/System.map /mnt/gentoo/boot/System.map-${kver}
 
 printf 'Installing device tree binaries ...\n'
-mkdir -p output/efi-part/overlays
-cp -au linux/arch/arm64/boot/dts/${device_tree} output/efi-part/
-cp -au \
-    linux/arch/arm64/boot/dts/overlays/*.dtb \
-    linux/arch/arm64/boot/dts/overlays/*.dtbo \
-    output/efi-part/overlays/
+mkdir -p "${O}"/efi-part/overlays
+cp -u "${O}"/linux/arch/arm64/boot/dts/${device_tree} "${O}"/efi-part/
+cp -u \
+    "${O}"/linux/arch/arm64/boot/dts/overlays/*.dtb \
+    "${O}"/linux/arch/arm64/boot/dts/overlays/*.dtbo \
+    "${O}"/efi-part/overlays/