devicepkg-dev: Make downstreamkernel_prepare callable from any folder

Removes the need to cd $builddir before calling downstreamkernel_prepare
in a kernel APKBUILD prepare function.

main/devicepkg-dev/APKBUILD

 pkgname="devicepkg-dev"
 pkgver=0.4
-pkgrel=0
+pkgrel=1
 pkgdesc="Provides default device package functions"
 url="https://postmarketos.org"
 arch="all"
@@ -25,4 +25,4 @@ package() {
 sha512sums="d69930dd790b00fb39760a37d95a10899f0d167e10e2804feb05d9ce04f94185dc32d36edc90214aba2ea2aa09bf18f7dab93f1d2eff23f67beb2cc83be30e7c  compiler-gcc.h
 638d50e6388eabf0da6bf0cff2fe9719ad8a808946f0077228db57fa13a26d9eeb39c1f2689c9a6f93ff9b3bcfdcfb7c358b180bba90e5bba8b9a9e78d25ed18  devicepkg_build.sh
 c732792596f56860f6ab9ddd53b9a7a80224400dd20097b20cebe17a6e7330e9178783f09db16132a28a555f83e29ef3643bfe069638b62998912a9a7ffefdc0  devicepkg_package.sh
-18de8ed6aa8aefea7c40b817023c276c3e63024086cde243c4c4fbe36e42fe51440c21dcded6b4f342054f59046c198d9c9775059b32f8a7cf822793d855ef4d  downstreamkernel_prepare.sh"
+ea0b95f2676287e6e1d348c38446babcf2f677dd12c71aaee8f82a2aa1b7706efeed658f1e09b59a3663c76973bf84fe278f429c157e6cd0c4cd6a5aa0cbed81  downstreamkernel_prepare.sh"

main/devicepkg-dev/downstreamkernel_prepare.sh

@@ -17,8 +17,8 @@ fi
 cp -v "/usr/share/devicepkg-dev/compiler-gcc.h" "$builddir/include/linux/"
 
 # Remove -Werror from all makefiles
-makefiles="$(find . -type f -name Makefile)
-	$(find . -type f -name Kbuild)"
+makefiles="$(find "$builddir" -type f -name Makefile)
+	$(find "$builddir" -type f -name Kbuild)"
 for i in $makefiles; do
 	sed -i 's/-Werror-/-W/g' "$i"
 	sed -i 's/-Werror//g' "$i"
@@ -26,4 +26,4 @@ done
 
 # Prepare kernel config ('yes ""' for kernels lacking olddefconfig)
 cp "$srcdir/$_config" "$builddir"/.config
-yes "" | make ARCH="$_carch" HOSTCC="$HOSTCC" oldconfig
+yes "" | make -C "$builddir" ARCH="$_carch" HOSTCC="$HOSTCC" oldconfig