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>

 # Template file for a debug utility

 # Put your download code here

 do_debug_foobar_get() {

     # For example:

     # CT_GetFile "foobar-${CT_FOOBAR_VERSION}" http://foobar.com/releases/

     :

 }

 # Put your extract code here

 do_debug_foobar_extract() {

     # For example:

     # CT_Extract "foobar-${CT_FOOBAR_VERSION}"

     # CT_Patch "foobar" "${CT_FOOBAR_VERSION}"

     :

 }

 # Put your build code here

 do_debug_foobar_build() {

     # For example:

     # mkdir -p "${CT_BUILD_DIR}/build-foobar"

     # CT_Pushd "${CT_BUILD_DIR}/build-foobar"

     # CT_DoExecLog CFG                                        \

     # "${CT_SRC_DIR}/foobar-${CT_FOOBAR_VERSION}/configure"   \

     #     --build=${CT_BUILD}                                 \

     #     --host=${CT_TARGET}                                 \

     #     --prefix=/usr                                       \

     #     --foobar-options

     # CT_DoExecLog ALL make

     # CT_DoExecLog ALL make DESTDIR="${CT_SYSROOT_DIR}" install

     # CT_Popd

     :

 }