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>

toolchain/toolchain-external/toolchain-external.mk

@@ -6,9 +6,10 @@
 
 #
 # This package implements the support for external toolchains, i.e
-# toolchains that have not been produced by Buildroot itself and that
-# Buildroot can download from the Web or that are already available on
-# the system on which Buildroot runs. So far, we have tested this
+# toolchains that are available pre-built, ready to use. Such toolchain
+# may either be readily available on the Web (Linaro, Sourcery
+# CodeBench, from processor vendors) or may be built with tools like
+# Crosstool-NG or Buildroot itself. So far, we have tested this
 # with:
 #
 #  * Toolchains generated by Crosstool-NG