build: Implement CONFIGDIR setting

In effort to support different builds of Aimee OS using the same
scripts, without necessarily having to fork this repository, the build
system now supports a `CONFIGDIR` setting.  When this variable is set,
files defining the target environment, such as the lists of packages to
install, the kernel configuration, the Portage configuration, etc. are
found in the path it specifes.

The reference build, for the Home Assistant Yellow board, is configured
in the `yellow` directory.  To build it, run:

```sh
CONFIGDIR=yellow ./vm-build.sh
```

repos/aimee-os/sys-boot/u-boot/u-boot-2023.01.ebuild

@@ -0,0 +1,71 @@
+# Copyright 1999-2023 Gentoo Authors
+# Distributed under the terms of the GNU General Public License v2
+
+EAPI=8
+
+inherit savedconfig toolchain-funcs
+
+MY_P="u-boot-${PV/_/-}"
+DESCRIPTION="Das U-Boot firmware"
+HOMEPAGE="http://www.denx.de/wiki/U-Boot"
+SRC_URI="https://ftp.denx.de/pub/u-boot/${MY_P}.tar.bz2"
+S="${WORKDIR}/${MY_P}"
+
+LICENSE="GPL-2"
+SLOT="0"
+KEYWORDS="~arm64"
+
+PATCHES=(
+	"${FILESDIR}"/0001-drivers-bcm283x-don-t-explicitly-disable-init.patch
+	"${FILESDIR}"/0001-rpi-Use-CONFIG_OF_BOARD-instead-of-CONFIG_EMBED.patch
+	"${FILESDIR}"/0002-drivers-bcm283x-allow-to-spawn-any-PL011-UART.patch
+	"${FILESDIR}"/0002-rpi-add-NVMe-to-boot-order.patch
+	"${FILESDIR}"/0003-Revert-nvme-Correct-the-prps-per-page-calculation-me.patch
+	"${FILESDIR}"/0004-usb-xhci-brcm-Make-driver-compatible-with-downstream.patch
+	"${FILESDIR}"/0005-nvme-improve-readability-of-nvme_setup_prps.patch
+	"${FILESDIR}"/0006-nvme-Use-pointer-for-CPU-addressed-buffers.patch
+	"${FILESDIR}"/0007-nvme-translate-virtual-addresses-into-the-bus-s-addr.patch
+)
+
+src_configure() {
+	restore_config .config
+	if [ -f .config ]; then
+		emake -j1 olddefconfig
+	else
+		# TODO Build images for other boards based on USE flags
+		emake -j1 rpi_4_defconfig
+	fi
+
+	tc-export AR BUILD_CC CC LD OBJCOPY PKG_CONFIG
+	tc-export_build_env
+}
+
+src_compile() {
+	unset KBUILD_OUTPUT KBUILD_SRC
+
+	local myemakeargs=(
+		V=1
+		AR="${AR}"
+		CC="${CC}"
+		HOSTCC="${BUILD_CC}"
+		HOSTCFLAGS="${BUILD_CFLAGS} ${BUILD_CPPFLAGS}"' $(HOSTCPPFLAGS)'
+		HOSTLDFLAGS="${BUILD_LDFLAGS}"
+		LD="${LD}"
+		OBJCOPY="${OBJCOPY}"
+	)
+
+	emake "${myemakeargs[@]}" \
+		NO_SDL=1 \
+		HOSTSTRIP=: \
+		STRIP=: \
+		CONFIG_ENV_OVERWRITE=y
+}
+
+src_test() { :; }
+
+src_install() {
+	save_config .config
+
+	insinto /usr/share/${PN}
+	doins u-boot.bin
+}