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.

Makefile

@@ -1,6 +1,9 @@
-update.tar: output/update.tar.zstd
+O ?= .
+IMAGESDIR ?= $(O)/images
 
-.prepared: \
+update.tar: $(IMAGESDIR)/update.tar.zstd
+
+$(O)/.prepared: \
 		prepare.sh \
 		config-portage.sh \
 		setup-local-repo.sh \
@@ -9,86 +12,87 @@ update.tar: output/update.tar.zstd
 	./prepare.sh
 	./config-portage.sh
 	./setup-local-repo.sh
-	touch .prepared
+	mkdir -p $(O)
+	touch $(O)/.prepared
 
-prepare: .prepared
+prepare: $(O)/.prepared
 
-.host-tools: \
+$(O)/.host-tools: \
 		build-host-tools.sh \
-		.prepared
+		$(O)/.prepared
 	./build-host-tools.sh
-	touch .host-tools
+	touch $(O)/.host-tools
 
-host-tools: .host-tools
+host-tools: $(O)/.host-tools
 
-.built: \
+$(O)/.built: \
 		build.sh \
 		build-rootfs.sh \
-		linux/arch/arm64/boot/Image.gz \
-		.host-tools \
-		.prepared
+		$(O)/linux/arch/arm64/boot/Image.gz \
+		$(O)/.host-tools \
+		$(O)/.prepared
 	./build.sh
 	./build-rootfs.sh
-	touch .built
+	touch $(O)/.built
 
-images/rootfs.squashfs: \
+$(IMAGESDIR)/rootfs.squashfs: \
 		build-squashfs.sh \
 		squashfs.exclude \
-		.built
-	./build-squashfs.sh
+		$(O)/.built
+	./build-squashfs.sh "$(IMAGESDIR)"
 
-squashfs: images/rootfs.squashfs
+squashfs: $(IMAGESDIR)/rootfs.squashfs
 
-linux/arch/arm64/boot/Image.gz: \
+$(O)/linux/arch/arm64/boot/Image.gz: \
 		build-kernel.sh \
 		linux.config \
-		.host-tools \
-		.prepared
-	./build-kernel.sh
+		$(O)/.host-tools \
+		$(O)/.prepared
+	./build-kernel.sh "$(O)"
 
-kernel: linux/arch/arm64/boot/Image.gz
+kernel: $(O)/linux/arch/arm64/boot/Image.gz
 
-output/efi-part/EFI/BOOT/BOOTAA64.efi: \
+$(O)/efi-part/EFI/BOOT/BOOTAA64.efi: \
 		build-grub.sh \
 		grub.cfg \
-		.host-tools \
-		.prepared
-	./build-grub.sh
+		$(O)/.host-tools \
+		$(O)/.prepared
+	./build-grub.sh "$(O)"
 
-grub: output/efi-part/EFI/BOOT/BOOTAA64.efi
+grub: $(O)/efi-part/EFI/BOOT/BOOTAA64.efi
 
-output/efi-part/u-boot.bin: \
+$(O)/efi-part/u-boot.bin: \
 		build-uboot.sh \
 		u-boot.config
-	./build-uboot.sh
+	./build-uboot.sh "$(O)"
 
-uboot: output/efi-part/u-boot.bin
+uboot: $(O)/efi-part/u-boot.bin
 
-images/sdcard.img: \
+$(IMAGESDIR)/sdcard.img: \
 		genimage.cfg \
 		genimage.sh \
 		post-build.sh \
-		output/efi-part/u-boot.bin \
-		output/efi-part/EFI/BOOT/BOOTAA64.efi
-	./post-build.sh
-	./genimage.sh
+		$(O)/efi-part/u-boot.bin \
+		$(O)/efi-part/EFI/BOOT/BOOTAA64.efi
+	./post-build.sh "$(O)"
+	./genimage.sh "$(O)" "$(IMAGESDIR)"
 
-sdcard.img: images/sdcard.img
+sdcard.img: $(IMAGESDIR)/sdcard.img
 
-images/firmware.img: images/sdcard.img
+$(IMAGESDIR)/firmware.img: $(IMAGESDIR)/sdcard.img
 
-output/update.tar.zstd: \
-		images/rootfs.squashfs \
-		images/firmware.img \
+$(IMAGESDIR)/update.tar.zstd: \
+		$(IMAGESDIR)/rootfs.squashfs \
+		$(IMAGESDIR)/firmware.img \
 		install-update.sh \
-		.host-tools \
-		.prepared
-	./build-update.sh
+		$(O)/.host-tools \
+		$(O)/.prepared
+	./build-update.sh "$(IMAGESDIR)"
 
 clean:
 	git -C u-boot clean -fdx && git -C u-boot checkout -- .
-	rm -rf linux output images tmp
-	rm -f .prepared .host-tools
+	rm -rf $(O)/linux $(O)/output $(IMAGESDIR) $(O)/tmp
+	rm -f $(O)/.prepared $(O)/.host-tools
 
 .PHONY: \
 	grub \