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>

# MIPS specific config options

choice

    bool

    prompt "ABI"

config ARCH_mips_o32

    bool

    prompt "o32"

    depends on ARCH_32

    help

      This is the -mabi=32 gcc option.

config ARCH_mips_n32

    bool

    prompt "n32"

    depends on ARCH_64

    help

      This is the -mabi=n32 gcc option.

config ARCH_mips_n64

    bool

    prompt "n64"

    depends on ARCH_64

    help

      This is the -mabi=64 gcc option.

# Not supported on Linux:

# o64   : seems related to *BSD

# eabi  : seems related to bare-metal

endchoice

config ARCH_mips_ABI

    string

    default "32"    if ARCH_mips_o32

    default "n32"   if ARCH_mips_n32

    default "64"    if ARCH_mips_n64