grep: support Perl regular expression

grep: support Perl regular expression

Supporting Perl regular expression is disabled with '--disable-perl-regexp'
but the manpage of 'grep' shows the '-P' option and returns below error:
"/bin/grep.grep -o -P 'PATTERN' file
 /bin/grep.grep: support for the -P option is not compiled into this \
               --disable-perl-regexp binary"

Signed-off-by: Haiqing Bai <Haiqing.Bai@windriver.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>