Simplify Makefile rules

Export SRC_PATH to run the checker with an extern solution.
Export UTILS_PATH to build libosmem.so is built using utils from checker.
Breakdown 'check' rule into multiple smaller rules.

Signed-off-by: Alex Apostolescu <alexx.apostolescu@gmail.com>

tests/Makefile

-SRC_PATH ?= ../src
+export SRC_PATH ?= $(realpath ../src)
+export UTILS_PATH ?= $(realpath ../utils)
+
 CC = gcc
-CPPFLAGS = -I../utils
+CPPFLAGS = -I$(UTILS_PATH)
 CFLAGS = -fPIC -Wall -Wextra -g
 LDFLAGS = -L$(SRC_PATH)
 LDLIBS = -losmem
@@ -10,24 +12,26 @@ BUILDDIR = bin
 SRCS = $(sort $(wildcard $(SOURCEDIR)/*.c))
 BINS = $(patsubst $(SOURCEDIR)/%.c, $(BUILDDIR)/%, $(SRCS))
 
-.PHONY: all clean src check lint
-
-all: src $(BUILDDIR) $(BINS)
+.PHONY: all src clean_src bins clean_bins check lint
 
-$(BUILDDIR):
-	mkdir -p $(BUILDDIR)
-
-$(BUILDDIR)/%: $(SOURCEDIR)/%.c
-	$(CC) $(CPPFLAGS) $(CFLAGS) -o $@ $^ $(LDFLAGS) $(LDLIBS)
+all: src bins
 
 src:
 	make -C $(SRC_PATH)
 
-check:
+clean_src:
 	make -C $(SRC_PATH) clean
-	make clean
-	make -i SRC_PATH=$(SRC_PATH)
-	SRC_PATH=$(SRC_PATH) python run-tests.py -m
+
+bins: $(BUILDDIR) $(BINS)
+
+clean_bins:
+	-rm -f *~
+	-rm -f $(BINS)
+
+check:
+	make clean_src src
+	make clean_bins bins
+	python checker.py
 
 lint:
 	-cd .. && checkpatch.pl -f src/*.c tests/src/*.c
@@ -36,6 +40,8 @@ lint:
 	-cd .. && shellcheck checker/*.sh
 	-cd .. && pylint tests/*.py
 
-clean:
-	-rm -f *~
-	-rm -f $(BINS)
+$(BUILDDIR):
+	mkdir -p $(BUILDDIR)
+
+$(BUILDDIR)/%: $(SOURCEDIR)/%.c
+	$(CC) $(CPPFLAGS) $(CFLAGS) -o $@ $^ $(LDFLAGS) $(LDLIBS)