Avoid toolchain download when it is preinstalled

For configurations using a toolchain that is preinstalled on
the host, <pkg>_SITE and <pkg>_SOURCE variables must be kept
empty to avoid downloading any toolchain package.
The actual implementation has been proposed by Thomas Petazzoni.

Signed-off-by: GONZALEZ Laurent <br2@gezedo.com>
Reviewed-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Tested-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

toolchain/toolchain-external/toolchain-external.mk

@@ -348,6 +348,14 @@ TOOLCHAIN_EXTERNAL_SITE = $(dir $(call qstrip,$(BR2_TOOLCHAIN_EXTERNAL_URL)))
 TOOLCHAIN_EXTERNAL_SOURCE = $(notdir $(call qstrip,$(BR2_TOOLCHAIN_EXTERNAL_URL)))
 endif
 
+# In fact, we don't need to download the toolchain, since it is already
+# available on the system, so force the site and source to be empty so
+# that nothing will be downloaded/extracted.
+ifeq ($(BR2_TOOLCHAIN_EXTERNAL_PREINSTALLED),y)
+TOOLCHAIN_EXTERNAL_SITE =
+TOOLCHAIN_EXTERNAL_SOURCE =
+endif
+
 TOOLCHAIN_EXTERNAL_INSTALL_STAGING = YES
 
 ifeq ($(BR2_TOOLCHAIN_EXTERNAL_BLACKFIN_UCLINUX_2012R1)$(BR2_TOOLCHAIN_EXTERNAL_BLACKFIN_UCLINUX_2012R2),y)