linux: fix linux-% shortcut targets

linux-% shortcut targets (short for linux26-%) ignores the ouput dir
$(O) so that 'make O=output.arm linux-menuconfig' is actually run in the
default $(O) directory output/ and not in output.arm/. Fix by passing on
$(O) if set.

[Peter: Use EXTRAMAKEARGS]
Signed-off-by: Bjørn Forsman <bjorn.forsman@gmail.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>

linux/linux.mk

@@ -200,4 +200,4 @@ endif
 endif
 
 linux-%:
-	$(MAKE) $(subst linux-,linux26-,$@)
+	$(MAKE) $(EXTRAMAKEARGS) $(subst linux-,linux26-,$@)