make: Further improve build time

This commit sees several improvements to the build performance:

- Introduce cache.mk, which creates makefiles caching often used
  variables, per tree and per project.
- Define more variables with := enclosed in condistions, instead of
  defining them with ?=, because the RHS of ?= is expanded deferredly.
- Add more definitions for executables.
- Move some more specialized definitions out into specialized makefiles,
  notably htdocs.mk and tmpl.mk

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

make/tmpl.mk

@@ -1,4 +1,19 @@
 include $(MODDIR)/make/defs.mk
+
+# -- TMPL
+INSTALLATION_FILE_TYPES += TMPL
+BUILD_TMPLDIR            = $(TOPDIR)/tmpl
+LOCAL_TMPL              += $(filter-out $(DONT_INSTALL),$(wildcard *.tmpl))
+ifndef TMPL_SUBDIR
+  TMPL_SUBDIR           := $(shell $(PWD) | $(SED) '/.*\/tmpl\(\/\|$$\)/!d; s%.*/tmpl\(/\|$$\)%%')
+endif
+ifeq ($(TMPL_SUBDIR),)
+  INSTALL_TMPLDIR        = $(PROJECT_TMPLDIR)
+else
+  INSTALL_TMPLDIR        = $(PROJECT_TMPLDIR)/$(TMPL_SUBDIR)
+endif
+INSTALLED_TMPL          += $(addprefix $(INSTALL_TMPLDIR)/,$(LOCAL_TMPL))
+
 include $(MODDIR)/make/rules.mk
 
 all: