toolchain-external: clarify external toolchain description

External toolchain can also have been generated by Buildroot previously, as
the list that follows demonstrates. Rephrase the paragraph describing what an
external toolchain is as suggested by Thomas Petazzoni, to make it clearer.

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>