external-toolchain: mention MIPS and PowerPC CodeSourcery toolchains

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>

toolchain/external-toolchain/ext-tool.mk

@@ -7,7 +7,9 @@
 #
 #  * Toolchains generated by Crosstool-NG
 #  * Toolchains generated by Buildroot
-#  * ARM toolchains made available by Codesourcery
+#  * ARM, MIPS and PowerPC toolchains made available by
+#    Codesourcery. For the MIPS toolchain, the -muclibc variant isn't
+#    supported yet, only the default glibc-based variant is.
 #
 # The basic principle is the following
 #