build: include BUILD_VARIANT in PKG_BUILD_DIR

This changes the default PKG_BUILD_DIR to take BUILD_VARIANT into
account (if set), so that packages do not need to manually override
PKG_BUILD_DIR just to handle variants.

This also updates most base packages with variants to use the updated
default PKG_BUILD_DIR.

Signed-off-by: Jeffery To <jeffery.to@gmail.com>

package/kernel/lantiq/ltq-adsl-mei/Makefile

@@ -8,7 +8,6 @@ include $(INCLUDE_DIR)/kernel.mk
 
 PKG_NAME:=ltq-adsl-mei
 PKG_RELEASE:=1
-PKG_BUILD_DIR:=$(KERNEL_BUILD_DIR)/ltq-adsl-mei-$(BUILD_VARIANT)/
 
 PKG_MAINTAINER:=John Crispin <john@phrozen.org>
 PKG_CHECK_FORMAT_SECURITY:=0

package/kernel/lantiq/ltq-atm/Makefile

@@ -8,7 +8,6 @@ include $(INCLUDE_DIR)/kernel.mk
 
 PKG_NAME:=ltq-atm
 PKG_RELEASE:=2
-PKG_BUILD_DIR:=$(KERNEL_BUILD_DIR)/ltq-atm-$(BUILD_VARIANT)
 
 PKG_MAINTAINER:=John Crispin <john@phrozen.org>
 PKG_LICENSE:=GPL-2.0+

package/kernel/lantiq/ltq-deu/Makefile

@@ -8,7 +8,6 @@ include $(INCLUDE_DIR)/kernel.mk
 
 PKG_NAME:=ltq-deu
 PKG_RELEASE:=1
-PKG_BUILD_DIR:=$(KERNEL_BUILD_DIR)/ltq-deu-$(BUILD_VARIANT)
 
 PKG_MAINTAINER:=John Crispin <john@phrozen.org>
 PKG_LICENSE:=GPL-2.0+

package/kernel/lantiq/ltq-ptm/Makefile

@@ -9,7 +9,6 @@ include $(INCLUDE_DIR)/kernel.mk
 
 PKG_NAME:=ltq-ptm
 PKG_RELEASE:=1
-PKG_BUILD_DIR:=$(KERNEL_BUILD_DIR)/ltq-ptm-$(BUILD_VARIANT)
 
 PKG_MAINTAINER:=John Crispin <john@phrozen.org>
 PKG_LICENSE:=GPL-2.0+