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>

# Kernel options

menu "Operating System"

# Config option used throughout the config and code to determine wether

# we have a kernel or not (there might be different bare metal stuff)...

config BARE_METAL

    bool

config MINGW32

    bool

# Each target OS (aka kernel) that support shared libraries can select

# this, so the user can decide whether or not to build a shared library

# enabled toolchain

config KERNEL_SUPPORTS_SHARED_LIBS

    bool

config KERNEL

    string

config KERNEL_VERSION

    string

source "config.gen/kernel.in"

comment "Common kernel options"

config SHARED_LIBS

    bool

    prompt "Build shared libraries"

    depends on KERNEL_SUPPORTS_SHARED_LIBS

    default y

    help

      Say 'y' here, unless you don't want shared libraries.

      You might not want shared libraries if you're building for a target that

      don't support it (maybe some nommu targets, for example, or bare metal).

source "config.gen/kernel.in.2"

endmenu