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>

#!@@CT_bash@@

# What we need:

#  - the .config file

# Parse the tools' paths configuration

. "${CT_LIB_DIR}/paths.mk"

# We'll need the stdout later, save it

exec 7>&1

# Parse the common functions

. "${CT_LIB_DIR}/scripts/functions"

# Don't care about any log file

exec >/dev/null

rm -f "${tmp_log_file}"

# Parse the configuration file

. .config.2

# Parse architecture and kernel specific functions

. "${CT_LIB_DIR}/scripts/build/arch/${CT_ARCH}.sh"

. "${CT_LIB_DIR}/scripts/build/kernel/${CT_KERNEL}.sh"

# Build CT_TARGET

CT_DoBuildTargetTuple

# All this for this single echo... :-( Sigh, I'll have to re-arrange things...

echo "${CT_TARGET}" >&7