flashrom: add BUILD_VARIANT to PKG_BUILD_DIR

Different variants will not be built right if they all share the same
build directory.

Signed-off-by: Eneas U de Queiroz <cotequeiroz@gmail.com>

utils/flashrom/Makefile

@@ -14,7 +14,7 @@ PKG_RELEASE:=1
 PKG_SOURCE:=$(PKG_NAME)-v$(PKG_VERSION).tar.bz2
 PKG_SOURCE_URL:=https://download.flashrom.org/releases
 PKG_SOURCE_SUBDIR:=$(PKG_NAME)-$(PKG_VERSION)
-PKG_BUILD_DIR:=$(BUILD_DIR)/$(PKG_NAME)-v$(PKG_VERSION)
+PKG_BUILD_DIR=$(BUILD_DIR)/$(PKG_NAME)-$(BUILD_VARIANT)/$(PKG_NAME)-v$(PKG_VERSION)
 PKG_HASH:=e1f8d95881f5a4365dfe58776ce821dfcee0f138f75d0f44f8a3cd032d9ea42b
 
 PKG_MAINTAINER:=Álvaro Fernández Rojas <noltari@gmail.com>