build: correct minor toolchain documentation error

Default values for toolchain tools are instead provided by the main
toolchain makefile, rather than a parent makefile. This was an oversight
from a previous version of the original toolchain refactor patch.

Change-Id: I75752ed7874b36e1c679d94292a2664e234c484b
Signed-off-by: Chris Kay <chris.kay@arm.com>
1 file changed