pmbootstrap build --src: override source for any package (#1210)

* New "pmbootstrap build --src=/local/source/path hello-world" syntax
* The local source path gets mounted inside the chroot
* From there, a copy of the source code gets created with rsync (so
  we can write into the source folder if necessary, for better
  compatibility with all kinds of APKBUILDs)
* After the aport gets copied into the chroot before building (as
  usually), we extend the APKBUILD with overrides to make it use
  mountpoint's source instead of downloading the package's source
  from the web as usually
* The package built with the local source gets _pYYYYMMDDHHMMSS
  appended to the pkgver
* linux-postmarketos-mainline: use $builddir, fix patch checksum

main/linux-postmarketos-mainline/APKBUILD

@@ -9,7 +9,7 @@ _kernver=${pkgver%_rc*}
 _mainver=${_kernver%.*}
 _patchlevel=${_kernver/$_mainver./}
 _basever=${_mainver}.$((_patchlevel-1))
-pkgrel=3
+pkgrel=4
 
 arch="x86_64 armhf aarch64"
 pkgdesc="Linux for pmOS supported chipsets (mainline, more bleeding-edge than stable)"
@@ -43,11 +43,11 @@ esac
 HOSTCC="${CC:-gcc}"
 HOSTCC="${HOSTCC#${CROSS_COMPILE}}"
 
-ksrcdir="$srcdir/linux-$_basever"
+builddir="$srcdir/linux-$_basever"
 
 prepare() {
 	local _patch_failed=
-	cd "$ksrcdir"
+	cd "$builddir"
 	# first apply patches in specified order
 	for i in $source; do
 		case $i in
@@ -70,7 +70,7 @@ prepare() {
 
 	mkdir -p "$srcdir"/build
 	cp -v "$srcdir"/$_config "$srcdir"/build/.config
-	make -C "$ksrcdir" O="$srcdir"/build ARCH="$_carch" HOSTCC="$HOSTCC" \
+	make -C "$builddir" O="$srcdir"/build ARCH="$_carch" HOSTCC="$HOSTCC" \
 		olddefconfig
 }
 
@@ -136,7 +136,7 @@ dev() {
 	# external modules, and create the scripts
 	mkdir -p "$dir"
 	cp "$srcdir"/$_config "$dir"/.config
-	make -j1 -C "$ksrcdir" O="$dir" ARCH="$_carch" HOSTCC="$HOSTCC" \
+	make -j1 -C "$builddir" O="$dir" ARCH="$_carch" HOSTCC="$HOSTCC" \
 		olddefconfig prepare modules_prepare scripts
 
 	# needed for 3rd party modules
@@ -154,7 +154,7 @@ dev() {
 	# this is taken from ubuntu kernel build script
 	# http://kernel.ubuntu.com/git/ubuntu/ubuntu-zesty.git/tree/debian/rules.d/3-binary-indep.mk
 
-	cd "$ksrcdir"
+	cd "$builddir"
 	find . -path './include/*' -prune \
 		-o -path './scripts/*' -prune -o -type f \
 		\( -name 'Makefile*' -o -name 'Kconfig*' -o -name 'Kbuild*' -o \