mdadm: Do not check RUN_DIR

Fixes build failure on hosts that do not have mdadm
installed/configured:

make[3]: Entering directory
`/local/users/fainelli/openwrt/trunk/build_dir/target-mipsel-linux-gnu_glibc/mdadm-4.0'
***** Parent of /run/mdadm does not exist.  Maybe set different RUN_DIR=
*****  e.g. make RUN_DIR=/dev/.mdadm
***** or set CHECK_RUN_DIR=0
make[3]: *** [check_rundir] Error 1
make[3]: Leaving directory
`/local/users/fainelli/openwrt/trunk/build_dir/target-mipsel-linux-gnu_glibc/mdadm-4.0'
make[2]: ***
[/local/users/fainelli/openwrt/trunk/build_dir/target-mipsel-linux-gnu_glibc/mdadm-4.0/.built]
Error 2
make[2]: Leaving directory
`/local/users/fainelli/openwrt/trunk/package/utils/mdadm'
make[1]: *** [package/utils/mdadm/compile] Error 2
make[1]: Leaving directory `/local/users/fainelli/openwrt/trunk'
make: *** [package/mdadm/compile] Error 2

Fixes: 980c41f8e0 ("utils/mdadm: Update to 4.0")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>

package/utils/mdadm/Makefile

@@ -9,7 +9,7 @@ include $(TOPDIR)/rules.mk
 
 PKG_NAME:=mdadm
 PKG_VERSION:=4.0
-PKG_RELEASE:=1
+PKG_RELEASE:=2
 
 PKG_SOURCE_URL:=@KERNEL/linux/utils/raid/mdadm
 PKG_SOURCE:=$(PKG_NAME)-$(PKG_VERSION).tar.xz
@@ -50,6 +50,7 @@ define Build/Compile
 		CC="$(TARGET_CC)" \
 		CFLAGS="$(TARGET_CFLAGS) -DHAVE_STDINT_H" \
 		LDFLAGS="$(TARGET_LDFLAGS)" \
+		CHECK_RUN_DIR=0 \
 		mdadm
 endef