Speed up rootfile generation

The old usage of find walked through the entire filesystem tree
and excluded some paths from being printed. The more efficient
solution is to skip walking through excluded directories entirely.

This is a slight speedup of the build process by a few minutes.

Signed-off-by: Michael Tremer <michael.tremer@ipfire.org>

lfs/Config

@@ -66,8 +66,9 @@ DIR_TMP     = /tmp
 #
 define FIND_FILES
 	cd $(ROOT)/ && find -mount \
-	-not -path './tools*' -not -path './tmp*' -not -path './usr/src*' -not -path './run/*' \
-	-not -path './dev*' -not -path './proc*' -not -path './install*' | sort
+		\( -path './tools' -or -path './tmp' -or -path './usr/src' \
+		-or -path './run' -or -path './dev' -or -path './proc' \
+		-or -path './install' \) -prune -or -print | sort
 endef
 
 # This is common starting logic for builds.