config: fix generated config files when GREP_OPTIONS set --color=always

grep is used when generating some .in files (in config.gen). If GREP_OPTIONS
contains --color=always, then the generated files will contains some escape
sequences setting the color, and mconf would choke on those escape sequences.

Always set and export an empty GREP_OPTIONS to avoid that.

Issue reported by Kevin Kirkup <kevin dot kirkup at gmail dot com> :
http://sourceware.org/ml/crossgcc/2009-12/msg00026.html

# AVR32 specific configuration file

# depends on EXPERIMENTAL

config ARCH_avr32

    select ARCH_USE_MMU

    select ARCH_DEFAULT_BE

    select ARCH_SUPPORT_ARCH

    select ARCH_SUPPORT_CPU

    select ARCH_SUPPORT_TUNE

    select ARCH_SUPPORT_FPU

     help

      The AVR32 architecture, as defined by:

        http://www.atmel.com/products/avr32