menu: visually split Shells from Utilities

As discussed in [1], add comments to distinghuish shells from utilities in
the 'Shells and Utilities' menu.

[1] http://lists.busybox.net/pipermail/buildroot/2013-December/083836.html



Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>