dchkernel.eclass: Add eclass for kernel building

Rather than duplicate the src_* functions in every kernel package, the
`dchkernel` eclass can be inherited to automatically define the
necessary functions to build a kernel using the configuration file in an
ebuild's $FILESDIR

eclass/dchkernel.eclass

@@ -0,0 +1,53 @@
+# Copyright 1999-2017 Dustin C. Hatch
+# Distributed under the terms of the GNU General Public License v2
+
+# @ECLASS: dchkernel.eclass
+# @MAINTAINER:
+# Dustin C. Hatch <dustin@hatch.name>
+# @AUTHOR:
+# Dustin C. Hatch <dustin@hatch.name>
+# @BLURB: Build dustin's kernels
+
+inherit multilib
+
+DEPEND="=sys-kernel/gentoo-sources-${PVR}"
+
+EXPORT_FUNCTIONS src_unpack src_configure src_install
+
+
+KSRC_DIR=/usr/src/linux-${PV}-gentoo
+if [ ${PR} != r0 ]; then
+	KSRC_DIR=${KSRC_DIR}-${PR}
+fi
+
+# @FUNCTION: dchkernel-src_unpack
+# @USAGE:
+# @DESCRIPTION:
+# Creates ${S}
+dchkernel_src_unpack() {
+	mkdir -p "${S}" || die
+}
+
+# @FUNCTION: dchkernel-src_configure
+# @USAGE:
+# @DESCRIPTION:
+# Copies the kernel configuration from ${FILESDIR} and runs make oldconfig
+dchkernel_src_configure() {
+	cp "${FILESDIR}"/${P}.kconfig .config || die
+	unset ARCH
+	make -C "${KSRC_DIR}" O="${S}" oldnoconfig || die
+}
+
+# @FUNCTION: dchkernel-src_install
+# @USAGE:
+# @DESCRIPTION:
+# Installs the kernel image, configuration file, and System.map into
+# /usr/${libdir}/kernels/${PF}
+dchkernel_src_install() {
+	local installpath="/usr/$(get_libdir)"/kernels/"${PF}"
+	dodir "${installpath}"
+	make install INSTALL_PATH="${D%/}${installpath}" || die
+	if in_iuse modules && use modules; then
+		make modules_install INSTALL_MOD_PATH="${D%/}" || die
+	fi
+}