memory_driver: Support overriding kernel directory

Support compiling the module against a different kernel version than the
currently running one by allowing to set either KVER or KDIR variables
on the make commandline.

Also modernize the makefile slightly and make use of the kernel's
'clean' target to ensure to remove all generated files.

Signed-off-by: Mathias Krause <minipli@grsecurity.net>

memory_driver/Makefile

@@ -8,9 +8,14 @@
 # MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 # GNU General Public License for more details.
 #
+ifneq ($(KERNELRELEASE),)
 obj-m := crash.o
+else
 PWD := $(shell pwd)
+KVER ?= $(shell uname -r)
+KDIR ?= /lib/modules/${KVER}/build
 all:
-	make -C /lib/modules/`uname -r`/build M=${PWD} SUBDIRS=${PWD} modules
+	${MAKE} -C ${KDIR} M=${PWD} SUBDIRS=${PWD} modules
 clean:
-	rm -f *.mod.c *.ko *.o Module.*
+	test -e ${KDIR}/Makefile && ${MAKE} -C ${KDIR} M=${PWD} SUBDIRS=${PWD} clean || ${RM} *.mod.c *.ko *.o Module.*
+endif