pkg-generic.mk: handle packages under toolchain

E.G. for toolchain-buildroot / toolchain-external. Now these packages are
correctly handled by make source / external-deps.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/pkg-generic.mk

@@ -509,6 +509,8 @@ ifeq ($(1),linux)
 $(2)_KCONFIG_VAR = BR2_LINUX_KERNEL
 else ifeq ($(4),boot/)
 $(2)_KCONFIG_VAR = BR2_TARGET_$(2)
+else ifeq ($(4),toolchain/)
+$(2)_KCONFIG_VAR = BR2_$(2)
 else
 $(2)_KCONFIG_VAR = BR2_PACKAGE_$(2)
 endif