Makefile: Minor cleanup and only set ROOTDIR if unset already.

The ROOTDIR variable is used for pretty formatting during compilation.
Some users might want to have this display the build from the top of
their tree instead of the finit build tree.

Signed-off-by: Joachim Nilsson <troglobit@gmail.com>

Makefile

@@ -26,7 +26,7 @@
 	uninstall-exec uninstall-data uninstall-dev
 
 # Top directory for building complete system, fall back to this directory
-ROOTDIR     = $(shell pwd)
+ROOTDIR    ?= $(shell pwd)
 
 #VERSION    ?= $(shell git tag -l | tail -1)
 VERSION    ?= 1.7
@@ -158,9 +158,6 @@ dev: distclean
 	  cd - >/dev/null; mv $$dir/$(DEV).tar.xz ../; cd ..;        \
 	  rm -rf $$dir; md5sum $(DEV).tar.xz | tee $(DEV).tar.xz.md5)
 
-# Include automatically generated rules, such as:
-# uncgi.o: .../some/dir/uncgi.c /usr/include/stdio.h
-# but don't bother during clean!
 ifneq ($(MAKECMDGOALS),clean)
 ifneq ($(MAKECMDGOALS),distclean)
 -include $(DEPS)