toolchain/toolchain-external: Allow relative paths in BR2_TOOLCHAIN_EXTERNAL_PATH.

Convert BR2_TOOLCHAIN_EXTERNAL_PATH to an absolute path when used.

Otherwise the symbolic links to the external toolchain binaries are
not installed in host/bin when BR2_TOOLCHAIN_EXTERNAL_PATH is relative.
This happens because TOOLCHAIN_EXTERNAL_INSTALL_WRAPPER
changes directory into host/bin to create the symbolic links.
From there the tools are no longer found via the relative path and
a single symbolic link host/bin/$(prefix)-* is created instead.

Although relative paths sounds like something less than ideal to put in
a Buildroot configuration, it's actually rather typical to put the
buildroot sources as a submodule (or subdirectory) of custom sources
(either in a BR2_EXTERNAL or not), in which case the relative path is
well-defined.

Signed-off-by: Juergen Stuber <juergen@jstuber.net>
Reviewed-by: Romain Naour <romain.naour@smile.fr>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>

toolchain/toolchain-external/pkg-toolchain-external.mk

@@ -68,7 +68,7 @@ TOOLCHAIN_EXTERNAL_DOWNLOAD_INSTALL_DIR = $(HOST_DIR)/opt/ext-toolchain
 ifeq ($(BR2_TOOLCHAIN_EXTERNAL_DOWNLOAD),y)
 TOOLCHAIN_EXTERNAL_INSTALL_DIR = $(TOOLCHAIN_EXTERNAL_DOWNLOAD_INSTALL_DIR)
 else
-TOOLCHAIN_EXTERNAL_INSTALL_DIR = $(call qstrip,$(BR2_TOOLCHAIN_EXTERNAL_PATH))
+TOOLCHAIN_EXTERNAL_INSTALL_DIR = $(abspath $(call qstrip,$(BR2_TOOLCHAIN_EXTERNAL_PATH)))
 endif
 
 ifeq ($(TOOLCHAIN_EXTERNAL_INSTALL_DIR),)