toolchain: fix the helpers.mk comment

This commit fixes various typos and mistakes in the comment at the top
of the helpers.mk file:

 * usefull -> useful

 * The optional stripping of libraries no longer exists, so there's no
   reason to mention it.

 * Indicate that the copy_toolchain_lib_root function is also used by
   the glibc package, not only by the external toolchain logic.

 * Separate more clearly the top comment introducing the entire file,
   from the comment introducing the first function.

[Peter: reword top comment, add missing 'by']
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>