kbuild: disable depmod in cross-compile kernel build

When building embedded systems in a cross-compile environment and
populating a target's file system image, we don't want to run the
depmod on the host as we may be building for a completely different
architecture. Since there's no such thing as a cross-depmod, we
just disable running depmod in the cross-compile case and we just
run depmod on the target at bootup.

Inspired by patches from Christian, Armin and Deepak.

This solves: http://bugzilla.kernel.org/show_bug.cgi?id=3881

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Cc: Christian Bjølevik <nafallo@magicalforest.se>
Cc: Deepak Saxena <dsaxena@mvista.com> and
Cc: Armin Kuster <akuster@mvista.com>,

Makefile

@@ -1030,19 +1030,12 @@ _modinst_:
 	fi
 	$(Q)$(MAKE) -f $(srctree)/scripts/Makefile.modinst
 
-# If System.map exists, run depmod.  This deliberately does not have a
-# dependency on System.map since that would run the dependency tree on
-# vmlinux.  This depmod is only for convenience to give the initial
+# This depmod is only for convenience to give the initial
 # boot a modules.dep even before / is mounted read-write.  However the
 # boot script depmod is the master version.
-ifeq "$(strip $(INSTALL_MOD_PATH))" ""
-depmod_opts	:=
-else
-depmod_opts	:= -b $(INSTALL_MOD_PATH) -r
-endif
 PHONY += _modinst_post
 _modinst_post: _modinst_
-	if [ -r System.map -a -x $(DEPMOD) ]; then $(DEPMOD) -ae -F System.map $(depmod_opts) $(KERNELRELEASE); fi
+	$(call cmd,depmod)
 
 else # CONFIG_MODULES
 
@@ -1259,15 +1252,6 @@ _emodinst_:
 	$(Q)mkdir -p $(MODLIB)/$(install-dir)
 	$(Q)$(MAKE) -f $(srctree)/scripts/Makefile.modinst
 
-# Run depmod only is we have System.map and depmod is executable
-quiet_cmd_depmod = DEPMOD  $(KERNELRELEASE)
-      cmd_depmod = if [ -r System.map -a -x $(DEPMOD) ]; then \
-                      $(DEPMOD) -ae -F System.map             \
-                      $(if $(strip $(INSTALL_MOD_PATH)),      \
-		      -b $(INSTALL_MOD_PATH) -r)              \
-		      $(KERNELRELEASE);                       \
-                   fi
-
 PHONY += _emodinst_post
 _emodinst_post: _emodinst_
 	$(call cmd,depmod)
@@ -1516,6 +1500,16 @@ quiet_cmd_rmdirs = $(if $(wildcard $(rm-dirs)),CLEAN   $(wildcard $(rm-dirs)))
 quiet_cmd_rmfiles = $(if $(wildcard $(rm-files)),CLEAN   $(wildcard $(rm-files)))
       cmd_rmfiles = rm -f $(rm-files)
 
+# Run depmod only is we have System.map and depmod is executable
+# and we build for the host arch
+quiet_cmd_depmod = DEPMOD  $(KERNELRELEASE)
+      cmd_depmod = \
+	if [ -r System.map -a -x $(DEPMOD) -a "$(SUBARCH)" == "$(ARCH)" ]; then \
+		$(DEPMOD) -ae -F System.map                                     \
+		$(if $(strip $(INSTALL_MOD_PATH)), -b $(INSTALL_MOD_PATH) -r)   \
+		$(KERNELRELEASE);                                               \
+	fi
+
 
 a_flags = -Wp,-MD,$(depfile) $(KBUILD_AFLAGS) $(AFLAGS_KERNEL) \
 	  $(NOSTDINC_FLAGS) $(KBUILD_CPPFLAGS) \