documentation: mention we can't use the distro toolchain as external

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

docs/buildroot.html

@@ -778,6 +778,15 @@ $(ZLIB_DIR)/libz.a: $(ZLIB_DIR)/.configured
 
     </ul>
 
+    <p>We also do not support using the distribution toolchain (i.e
+    the gcc/binutils/C library installed by your distribution) as the
+    toolchain to build software for the target. This is because your
+    distribution toolchain is not a "pure" toolchain (i.e only with
+    the C/C++ library), so we cannot import it properly into the
+    Buildroot build environment. So even if you are building a system
+    for a x86 or x86_64 target, you have to generate a
+    cross-compilation toolchain with Buildroot or Crosstool-NG.</p>
+
     <h2 id="add_packages">Adding new packages to Buildroot</h2>
 
     <p>This section covers how new packages (userspace libraries or