Setup an external toolchain with Yocto

http://wiki.kaeilos.com/index.php/Setup_an_external_toolchain_with_Yocto
Setup an external toolchain with Yocto
You can use the sourcery g++ toolchain, but it requires custom tuning arguments to control selection of the multilib in the external toolchain sysroots, so you should switch to a separate layer and override the tuning files in question.
For example.
If you want to see an example of external toolchain stuff that works, see the meta-sourcery layer.
Watch this presentation about how to setup an external toolchain with oe-core.

Example with Linaro toolchain

#
# Setting the following variables will enable the use of the Linaro toolchain. If not set
#     you will build using gcc built by Yocto. EXTERNAL_TOOLCHAIN must be set to the path
#     where the linaro toolchain exists and should have subdirectories named lib, bin, etc
#

TCLIBC = "external-linaro-toolchain"
TCMODE = "external-linaro"
EXTERNAL_TOOLCHAIN = "/home/koan/altera-linux/linaro/gcc-linaro-arm-linux-gnueabihf-4.7-2012.11-20121123_linux"