build: add support for automatically removing build dir contents during build

This is used to save space on buildbot instances.
If any part of a package needs to be rebuild, the whole package is
rebuilt from scratch. Stamp files are preserved to allow dependency
checks to work

Signed-off-by: Felix Fietkau <nbd@nbd.name>

package/Makefile

@@ -10,6 +10,7 @@ curdir:=package
 include $(INCLUDE_DIR)/rootfs.mk
 
 -include $(TMP_DIR)/.packagedeps
+$(curdir)/autoremove:=1
 $(curdir)/builddirs:=$(sort $(package-) $(package-y) $(package-m))
 $(curdir)/builddirs-install:=.
 $(curdir)/builddirs-default:=. $(sort $(package-y) $(package-m))