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>

 # Menu for the cross GDB

 config STATIC_TOOLCHAIN

     select GDB_CROSS_STATIC if GDB_CROSS

 comment "Cross-gdb - disabled (requires expat; re-run ./configure to enable)"

     depends on !CONFIGURE_has_expat

 comment "Static cross-gdb - disabled (requires static expat; re-run ./configure to enable)"

     depends on STATIC_TOOLCHAIN && !CONFIGURE_has_static_expat

 config GDB_CROSS

     bool

     prompt "Cross-gdb"

     default y

     depends on CONFIGURE_has_expat

     depends on !STATIC_TOOLCHAIN || CONFIGURE_has_static_expat

     select GDB_GDBSERVER if ! BARE_METAL

     help

       Build and install a cross-gdb for the target, to run on host.

 if GDB_CROSS

 config GDB_CROSS_STATIC

     bool

     prompt "Build a static cross gdb"

     depends on CONFIGURE_static_link_ok

     depends on CONFIGURE_has_static_expat

     help

       A static cross gdb can be usefull if you debug on a machine that is

       not the one that is used to compile the toolchain.

       That way, you can share the cross-gdb without installing a toolchain

       on every machine that will be used to debug target programs.

 endif # GDB_CROSS