toolchain-external-custom: allow specifying relative path to binaries

toolchain-external-custom: allow specifying relative path to binaries

There are cases where a downloaded toolchain doesn't have its binaries
placed directly in a "bin" subfolder (where BuildRoot currently looks
for them).

A common example is the official Raspberry Pi Toolchain
(https://github.com/raspberrypi/tools), which has its binaries in
"arm-bcm2708/arm-linux-gnueabihf/bin".

This commit introduces BR2_TOOLCHAIN_EXTERNAL_REL_BIN_PATH that defaults
to "bin" and can be changed as needed.

Signed-off-by: Calin Crisan <ccrisan@gmail.com>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
[Thomas: rework a bit how TOOLCHAIN_EXTERNAL_REL_BIN_PATH is defined.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>