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 options for gcc

config CC_GCC_HAS_ARCH_OPTIONS

    default y

config CC_GCC_mips_llsc

    tristate

    prompt "Use llsc"

    default m

    help

      This configures how gcc will emit load-link, store-conditional and

      sync opcodes. Normally, you'd set this to 'M', and configure will do

      the correct thing (ie. enable llsc for Linux-based targets, disable

      for others). If you know better, you can force either using llsc or

      not using it (resp. 'Y' or 'N').

config CC_GCC_mips_synci

    tristate

    prompt "Use synci"

    default m

    help

      This configures how gcc will emit the synci opcode. Normally, you'd

      set this to 'M', and configure will do the correct thing (ie. disable).

      If you know better, you can force using synci (say 'Y'), or not using

      it (say 'N').

config CC_GCC_mips_plt

    bool

    prompt "Use copy relocations & PLTs"

    help

      This configures gcc to use copy relocations & PLTs. These are

      extensions to the traditional SVR4-based MIPS ABIs and require

      support from GNU binutils and the runtime C library.

      Say 'N' to keep the default and not use them, say 'Y' to use them.