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>

 # Compute ARM-specific values

 CT_DoArchTupleValues() {

     # The architecture part of the tuple:

     CT_TARGET_ARCH="${CT_ARCH}${target_endian_eb}"

     # The system part of the tuple:

     case "${CT_LIBC},${CT_ARCH_ARM_EABI}" in

         *glibc,y)   CT_TARGET_SYS=gnueabi;;

         uClibc,y)   CT_TARGET_SYS=uclibcgnueabi;;

         *,y)        CT_TARGET_SYS=eabi;;

     esac

     # Set the default instruction set mode

     case "${CT_ARCH_ARM_MODE}" in

         arm)    ;;

         thumb)

             CT_ARCH_CC_CORE_EXTRA_CONFIG="--with-mode=thumb"

             CT_ARCH_CC_EXTRA_CONFIG="--with-mode=thumb"

 #            CT_ARCH_TARGET_CFLAGS="-mthumb"

             ;;

     esac

     if [ "${CT_ARCH_ARM_INTERWORKING}" = "y" ]; then

         CT_ARCH_TARGET_CFLAGS+=" -mthumb-interwork"

     fi

 }