Config.in files: add/update comments on (e)glibc dependencies

This patch adds missing comments about (e)glibc dependencies and updates the
text of existing comments.

Additionally, it splits dependency expressions for the touched packages from
  depends on BR2_BASE_DEP && !BR2_TOOLCHAIN_USES_GLIBC
to
  depends on BR2_BASE_DEP
  depends on !BR2_TOOLCHAIN_USES_GLIBC
so that (positive) base dependencies are separate from the (negative)
toolchain dependencies. This strategy makes it easier to write such comments
(because one can simply copy the base dependency from the actual package
config option), but also avoids complex and long boolean expressions.

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