toolchain: configs: improve toolchain config readability

Add (or move) comment lines in toolchain-related Config.in files
to improve readability of the toolchain config section.

Source linux-headers/Config.in.host after toolchain-buildroot items.

For toolchain-buildroot case, the config file now looks like this:
*
* Toolchain
*

*
* Toolchain Buildroot Options
*

*
* Kernel Header Options
*

*
* uClibc Options
*

*
* Binutils Options
*

*
* GCC Options
*

*
* Host GDB Options
*

*
* Toolchain Generic Options
*

Signed-off-by: Petri Gynther <pgynther@google.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>