overlay coreos-base/common-oem-files: New package

Th package will generate and install both grub.cfg and oem-release
files into /usr/share/oem. Each platform can customize the process by
providing their own fragments for each of the two files if necessary.

sdk_container/src/third_party/coreos-overlay/coreos-base/common-oem-files/common-oem-files-0.ebuild

@@ -0,0 +1,86 @@
+# Copyright (c) 2023 The Flatcar Maintainers.
+# Distributed under the terms of the GNU General Public License v2
+
+EAPI=8
+
+OEMIDS=(
+)
+
+DESCRIPTION='Common OEM files'
+HOMEPAGE='https://www.flatcar.org/'
+
+LICENSE='Apache-2.0'
+SLOT='0'
+KEYWORDS='amd64 arm64'
+IUSE="${OEMIDS[*]}"
+REQUIRED_USE="^^ ( ${OEMIDS[*]} )"
+
+# No source directory.
+S="${WORKDIR}"
+
+DEPEND=""
+RDEPEND="${DEPEND}"
+BDEPEND="
+	app-portage/gentoolkit
+"
+
+src_compile() {
+    local oemid package ebuild version name homepage lines
+
+    for oemid in "${OEMIDS[@]}"; do
+        if use "${oemid}"; then break; fi
+    done
+
+    package="coreos-base/oem-${oemid}"
+    ebuild=$(equery which "${package}")
+    version=${ebuild##*"oem-${oemid}-"}
+    version=${version%%'.ebuild'}
+    if [[ -z "${version}" ]]; then
+        die "Could not deduce a version from ebuild ${ebuild##*/} (${ebuild})"
+    fi
+    name=$(source <(grep -F 'OEM_NAME=' "${ebuild}"); echo "${OEM_NAME}")
+    if [[ -z "${name}" ]]; then
+        die "Missing OEM_NAME variable in ${ebuild##*/}"
+    fi
+    # We need to prefix the HOMEPAGE variable with SYSEXT_, because
+    # portage marks HOMEPAGE as readonly and this gets propagated to
+    # subshells, so sourcing a snippet with HOMEPAGE=foo won't
+    # overwrite the readonly variable.
+    homepage=$(source <(grep -F 'HOMEPAGE=' "${ebuild}" | sed -e 's/^/SYSEXT_/'); echo "${SYSEXT_HOMEPAGE}")
+    lines=(
+        "ID=${oemid}"
+        "VERSION_ID=${version}"
+        "NAME=\"${name}\""
+    )
+    if [[ -n "${homepage}" ]]; then
+        lines+=( "HOME_URL=\"${homepage}\"" )
+    fi
+    lines+=(
+        'BUG_REPORT_URL="https://issues.flatcar.org"'
+    )
+
+    {
+        printf '%s\n' "${lines[@]}"
+        if [[ -e "${FILESDIR}/${oemid}/oem-release.frag" ]]; then
+            cat "${FILESDIR}/${oemid}/oem-release.frag"
+        fi
+    } >"${T}/oem-release"
+
+    lines=(
+        '# Flatcar GRUB settings'
+        ''
+        "set oem_id=\"${oemid}\""
+    )
+    {
+        printf '%s\n' "${lines[@]}"
+        if [[ -e "${FILESDIR}/${oemid}/grub.cfg.frag" ]]; then
+            cat "${FILESDIR}/${oemid}/grub.cfg.frag"
+        fi
+    } >"${T}/grub.cfg"
+}
+
+src_install() {
+    insinto "/oem"
+    doins "${T}/grub.cfg"
+    doins "${T}/oem-release"
+}

sdk_container/src/third_party/coreos-overlay/coreos-base/common-oem-files/metadata.xml

@@ -0,0 +1,4 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!DOCTYPE pkgmetadata SYSTEM "https://www.gentoo.org/dtd/metadata.dtd">
+<pkgmetadata>
+</pkgmetadata>