kbuild: pacman-pkg: move common commands to a separate function

All build and package functions share the following commands:

  export MAKEFLAGS="${KBUILD_MAKEFLAGS}"
  cd "${objtree}"

Factor out the common code.
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Acked-by:   Thomas Weißschuh <linux@weissschuh.net>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Reviewed-by: Christian Heusel <christian@heusel.eu>

scripts/package/PKGBUILD

@@ -36,11 +36,15 @@ makedepends=(
 )
 options=(!debug !strip !buildflags !makeflags)
 
-build() {
+_prologue() {
 	# MAKEFLAGS from makepkg.conf override the ones inherited from kbuild.
 	# Bypass this override with a custom variable.
 	export MAKEFLAGS="${KBUILD_MAKEFLAGS}"
 	cd "${objtree}"
+}
+
+build() {
+	_prologue
 
 	${MAKE} KERNELRELEASE="${KERNELRELEASE}" KBUILD_BUILD_VERSION="${pkgrel}"
 }
@@ -48,10 +52,10 @@ build() {
 _package() {
 	pkgdesc="The ${pkgdesc} kernel and modules"
 
-	export MAKEFLAGS="${KBUILD_MAKEFLAGS}"
-	cd "${objtree}"
 	local modulesdir="${pkgdir}/usr/${MODLIB}"
 
+	_prologue
+
 	echo "Installing boot image..."
 	# systemd expects to find the kernel here to allow hibernation
 	# https://github.com/systemd/systemd/commit/edda44605f06a41fb86b7ab8128dcf99161d2344
@@ -76,10 +80,10 @@ _package() {
 _package-headers() {
 	pkgdesc="Headers and scripts for building modules for the ${pkgdesc} kernel"
 
-	export MAKEFLAGS="${KBUILD_MAKEFLAGS}"
-	cd "${objtree}"
 	local builddir="${pkgdir}/usr/${MODLIB}/build"
 
+	_prologue
+
 	if grep -q CONFIG_MODULES=y include/config/auto.conf; then
 		echo "Installing build files..."
 		"${srctree}/scripts/package/install-extmod-build" "${builddir}"
@@ -100,8 +104,7 @@ _package-api-headers() {
 	provides=(linux-api-headers)
 	conflicts=(linux-api-headers)
 
-	export MAKEFLAGS="${KBUILD_MAKEFLAGS}"
-	cd "${objtree}"
+	_prologue
 
 	${MAKE} headers_install INSTALL_HDR_PATH="${pkgdir}/usr"
 }