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>

# 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