Support LOCAL_MODULE_RELATIVE_PATH

Most users of LOCAL_MODULE_PATH are setting a subdirectory of the
normal install path, for example to install HALs into system/lib/hw.
This is problematic for multiarch builds, where the install location
depends on the arch.  Allow modules to specify LOCAL_MODULE_RELATIVE_PATH.
HALs will generally use:
LOCAL_MODULE_RELATIVE_PATH := hw

Change-Id: I4e4ceec61d026bbe74ba604554c06104bde42e5e
diff --git a/core/clear_vars.mk b/core/clear_vars.mk
index 56e17b0..8863fe6 100644
--- a/core/clear_vars.mk
+++ b/core/clear_vars.mk
@@ -4,6 +4,7 @@
 
 LOCAL_MODULE:=
 LOCAL_MODULE_PATH:=
+LOCAL_MODULE_RELATIVE_PATH :=
 LOCAL_MODULE_STEM:=
 LOCAL_DONT_CHECK_MODULE:=
 LOCAL_CHECKED_MODULE:=