scripts/libc: do not build add-ons by default

Currently, no --enable-add-ons option is passed to libc configure when
"$(do_libc_add_ons_list ,)" is empty, which makes configure automatically search
for present add-ons. In that case, all present add-ons are built, although
no add-on was selected by the user in the config. Moreover, this can make the
configure fail if some non-standard add-ons like eglibc-localedef are present.

This behavior also leads to an inconsistency from a user point of view between
the following cases:
- LIBC_ADDONS_LIST="", LIBC_GLIBC_USE_PORTS=n and THREADS="none" in the config,
which makes "$(do_libc_add_ons_list ,)" return "", so all present add-ons
are built.
- LIBC_ADDONS_LIST="", LIBC_GLIBC_USE_PORTS=n and THREADS!="none" in the
config, which makes "$(do_libc_add_ons_list ,)" return the add-on supporting
the chosen threading implementation, e.g. "nptl", so only this add-on is
built.

This patch disables the building of all add-ons in that case.

It is still possible to build all present add-ons by adding --enable-add-ons to
LIBC_GLIBC_EXTRA_CONFIG_ARRAY.

Signed-off-by: "Benoît THÉBAUDEAU" <benoit.thebaudeau@advansee.com>

 reporter_name="Anthony Foiani <anthony.foiani@gmail.com>"

 reporter_url="http://sourceware.org/ml/crossgcc/2010-09/msg00100.html"

 reporter_comment="This is a sample config file for Freescale e500v2 processors (e.g., MPC8548,

 MPC8572). It uses eglibc (for e500/SPE patches) and a recent gcc (4.6.0,

 for e500v2 DPFP support) and will generate appropriate dual-precision

 floating point instructions by default.

 Note: If building a Linux kernel with this toolchain, you will want to make

 sure -mno-spe AND -mspe=no are passed to gcc to prevent SPE ABI/instructions

 from getting into the kernel (which is currently unsupported). At this time,

 the kernel build system properly passes those two options, but older kernels

 were only passing -mno-spe by default."