devicepkg-dev: Properly replace -Werror= (!181)

The 5.0 kernel now includes -Werror= parameters in the makefile, this
changes downstreamkernel_prepare.sh to support them. Without this, the
script will replace, for example, -Werror=blah with =blah, which causes
the kernel compilation to fail.

main/devicepkg-dev/downstreamkernel_prepare.sh

@@ -21,6 +21,7 @@ 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=/-W/g' "$i"
 	sed -i 's/-Werror//g' "$i"
 done