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>

toolchain/helpers.mk

@@ -1,9 +1,10 @@
-# This Makefile fragment declares helper functions, usefull to handle
-# non- buildroot-built toolchains, eg. purely external toolchains.
-#
-# Copy a toolchain library and its symbolic links from the sysroot
-# directory to the target directory. Also optionaly strips the
-# library.
+# This Makefile fragment declares toolchain related helper functions.
+
+# The copy_toolchain_lib_root function copies a toolchain library and
+# its symbolic links from the sysroot directory to the target
+# directory. Note that this function is used both by the external
+# toolchain logic, and the glibc package, so care must be taken when
+# changing this function.
 #
 # Most toolchains (CodeSourcery ones) have their libraries either in
 # /lib or /usr/lib relative to their ARCH_SYSROOT_DIR, so we search