toolchain-external: default BR2_TOOLCHAIN_EXTERNAL_PATH to empty

It makes no sense to default to an arbitrary path. In addition, it in
fact works correctly when it is empty. In that case, the toolchain will
be searched in PATH.

Update the help text to explain the above, and also that the compiler
is supposed to be in the bin subdirectory.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>