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-grub.sh

@@ -5,6 +5,8 @@ set -e
 
 . ./config
 
+O="${1}"
+
 GRUB_MODULES='
 boot
 echo
@@ -26,17 +28,17 @@ zstd
 '
 
 echo 'Creating GRUB image ...'
-mkdir -p output/efi-part/EFI/BOOT
+mkdir -p "${O}"/efi-part/EFI/BOOT
 grub-mkimage \
     -O arm64-efi \
-    -o output/efi-part/EFI/BOOT/BOOTAA64.efi \
+    -o "${O}"/efi-part/EFI/BOOT/BOOTAA64.efi \
     -d /usr/${target}/usr/lib/grub/arm64-efi \
     -p /EFI/gentoo \
     ${GRUB_MODULES}
 
 echo 'Generating GRUB configuration file ...'
-mkdir -p output/efi-part/EFI/gentoo
-cp -uv grub.cfg output/efi-part/EFI/gentoo
-grub-editenv output/efi-part/EFI/gentoo/grubenv set rootflags='ro'
-grub-editenv output/efi-part/EFI/gentoo/grubenv set default=0
-grub-editenv output/efi-part/EFI/gentoo/grubenv set timeout=5
+mkdir -p "${O}"/efi-part/EFI/gentoo
+cp -uv grub.cfg "${O}"/efi-part/EFI/gentoo
+grub-editenv "${O}"/efi-part/EFI/gentoo/grubenv set rootflags='ro'
+grub-editenv "${O}"/efi-part/EFI/gentoo/grubenv set default=0
+grub-editenv "${O}"/efi-part/EFI/gentoo/grubenv set timeout=5