Further improve build time: Try to avoid calling projects.py

This commit tries to remove the necessity to call projects.py from $(TOPDIR) to
speed up recursive builds over all projects yet again. This is a major
undertaking. There are two variables which are filled py projects.py in
$(TOPDIR): PREREQ and PREREQ_DIRS. Sadly, the latter is a path relative to
$(TOPDIR)/make, so this is kind of pointless. Unless the cache is maintained in
$(TOPDIR), a thing I tried to avoid. So this commit is only able to cache
$(PREREQ), not $(PREREQ_DIRS), which still is a hassle. Introduced defs-dirs.mk
for that, to make it accessible to make.mk, and modified all the other parts of
the machinery, too.

Signed-off-by: Jan Lindemann <jan@janware.com>

make/defs.mk

@@ -7,8 +7,19 @@ JW_BUILD_DEF_MK_INCLUDED = true
 
 MAKEFLAGS             += -r
 
--include $(MODDIR)/make/.cache.mk
--include $(TOPDIR)/make/.cache.mk
+define try_include
+  ifneq ($(wildcard $(1)),)
+    include $(1)
+  endif
+endef
+
+ifneq ($(wildcard $(MODDIR)/make/.cache.mk),)
+  include $(MODDIR)/make/.cache.mk
+endif
+ifneq ($(wildcard $(TOPDIR)/make/.cache.mk),)
+  include $(TOPDIR)/make/.cache.mk
+endif
+
 include $(MODDIR)/make/platform.mk
 include $(MODDIR)/make/projects.mk
 
@@ -87,11 +98,6 @@ ifndef PRINTF
   PRINTF              := $(shell $(WHICH) printf)
 endif
 
-# makefile helpers
-FIND_SUBDIRS          := $(filter-out . .. nomake CVS $(IGNORE_SUBDIRS),\
-	$(patsubst ./%,%,$(patsubst %/,%,$(dir $(shell $(FIND) . -maxdepth 2 -mindepth 2 -a \( -iname Makefile -o -iname GNUmakefile \))))))
-SUBDIRS_TO_ITERATE     = $(filter-out $(IGNORE_SUBDIRS),$(SUBDIRS))
-
 PKG_SH_EXE            := /bin/bash $(MOD_SCRIPT_DIR)/pkg.sh
 LOG_INSTALL_SH        := $(PKG_SH_EXE) log-install
 UPDATE_REPO_SH        := /opt/packager-server/bin/packager-server schedule-rebuild