qcacld-3.0: Allow out-of-tree build with relative path in M

The Make variable $(M) must point to the directory that contains the
module source code (which includes this Makefile). It can either be an
absolute or a relative path. If it is a relative path, then it must be
relative to the kernel source directory (KERNEL_SRC). An absolute path
can be obtained very easily through $(shell pwd). Generating a path
relative to KERNEL_SRC is difficult and we accept some outside help by
letting the caller override the variable $(M). Allowing a relative path
for $(M) enables us to have the build system put output/object files
(.o, .ko.) into a directory different from the module source directory.

Change-Id: Ic5dbcaa4579e4b7fe8b01399e03293c3bd425d27
CRs-Fixed: 2182672

Makefile

@@ -1,6 +1,18 @@
 KERNEL_SRC ?= /lib/modules/$(shell uname -r)/build
 
-KBUILD_OPTIONS := WLAN_ROOT=$(PWD)
+# The Make variable $(M) must point to the directory that contains the module
+# source code (which includes this Makefile). It can either be an absolute or a
+# relative path. If it is a relative path, then it must be relative to the
+# kernel source directory (KERNEL_SRC). An absolute path can be obtained very
+# easily through $(shell pwd). Generating a path relative to KERNEL_SRC is
+# difficult and we accept some outside help by letting the caller override the
+# variable $(M). Allowing a relative path for $(M) enables us to have the build
+# system put output/object files (.o, .ko.) into a directory different from the
+# module source directory.
+M ?= $(shell pwd)
+
+# WLAN_ROOT must contain an absolute path (i.e. not a relative path)
+KBUILD_OPTIONS := WLAN_ROOT=$(shell cd $(KERNEL_SRC); readlink -e $(M))
 KBUILD_OPTIONS += MODNAME?=wlan
 
 #By default build for CLD
@@ -11,10 +23,10 @@ KBUILD_OPTIONS += $(WLAN_SELECT)
 KBUILD_OPTIONS += $(KBUILD_EXTRA) # Extra config if any
 
 all:
-	$(MAKE) -C $(KERNEL_SRC) M=$(shell pwd) modules $(KBUILD_OPTIONS)
+	$(MAKE) -C $(KERNEL_SRC) M=$(M) modules $(KBUILD_OPTIONS)
 
 modules_install:
-	$(MAKE) INSTALL_MOD_STRIP=1 -C $(KERNEL_SRC) M=$(shell pwd) modules_install
+	$(MAKE) INSTALL_MOD_STRIP=1 M=$(M) -C $(KERNEL_SRC) modules_install
 
 clean:
-	$(MAKE) -C $(KERNEL_SRC) M=$(PWD) clean
+	$(MAKE) -C $(KERNEL_SRC) M=$(M) clean