build: reset ALTERNATIVES field in Package/Default

Otherwise ipkg packages may wrongly take on alternatives specs of
another package sharing the same package Makefile

Fixes FS#753

Signed-off-by: Yousong Zhou <yszhou4tech@gmail.com>

include/package-defaults.mk

@@ -56,6 +56,7 @@ define Package/Default
   VARIANT:=
   DEFAULT_VARIANT:=
   USERID:=
+  ALTERNATIVES:=
 endef
 
 Build/Patch:=$(Build/Patch/Default)