external toolchain: Only prefix path if specified

Only prefix the external toolchain calls with its absolute path if
BR2_TOOLCHAIN_EXTERNAL_PATH is set, otherwise just assume it will
be available in the path.

Signed-off-by: Sonic Zhang <sonic.zhang@analog.com>
Acked-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>