Remove --{enable, disable}-debug configure flags.

Adding this flag when BR2_ENABLE_DEBUG is activated make several
packages to produce binaries that do not work as expected (e.g., dhcp,
lame, nano). Moreover, the help message of BR2_ENABLE_DEBUG does not
say it is adding this flag. It is supposed to build packages with
debugging symbols enabled. So, let it do that only.

* package/Makefile.in: Do not add --{enable,disable}-debug flags.
* package/pkg-autotools.mk: Remove ENABLE_DEBUG as it is not set
  anymore.

Signed-off-by: Johan Oudinet <johan.oudinet@gmail.com>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>