config: add an option not to remove the destination directory

In certain circumstances, removing the destination/installation directory
is a bad idea. For example, when the build environment is already taking
care of sanitising the build tree, and pre-installs stuff in there, it is
a very bad idea to remove the destination directory.

This happens now in buildroot, as the crostool-NG backend now installs the
toolchain in the common host-tools directory, and pre-install there a few
host-utilities (eg. host-automake and host-gawk).

Provide a config knob to turn on/off the removal of the destination
directory, defaulting to 'y' (previous behavior), and forced to 'n' when
used as a backend.

Reported-by: Peter Korsgaard <jacmet@sunsite.dk>
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>

# newlib second-part options

config LIBC_NEWLIB_IO_C99FMT

    bool

    prompt "Enable IOs on C99 formats"

    default n

    help

      Enable support for IOs on C99 formats.

config LIBC_NEWLIB_IO_LL

    bool

    prompt "Enable IOs on long long"

    default n

    help

      Enable support for IOs on long long integers.

config LIBC_NEWLIB_IO_FLOAT

    bool

    prompt "Enable IOs on floats and doubles"

    default n

    help

      Enable support for IOs on floating point

      values (float and double).

config LIBC_NEWLIB_IO_LDBL

    bool

    prompt "Enable IOs on long doubles"

    default n

    depends on LIBC_NEWLIB_IO_FLOAT

    help

      Enable support for IOs on long doubles.