"env grep" - add support for regular expression matches

When CONFIG_REGEX is enabled, the new option "-e" becomes available
which causes regular expression matches to be used.  This allows for
example things like these:

- print all MAC addresses:

	=> env grep -e eth.*addr
	eth1addr=00:10:ec:80:c5:15
	ethaddr=00:10:ec:00:c5:15

- print all variables that have at least 2 colons in their value:

	=> env grep -v -e :.*:
	addip=setenv bootargs ${bootargs} ip=${ipaddr}:${serverip}:${gatewayip}:${netmask}:${hostname}:${netdev}:off
	panic=1
	eth1addr=00:10:ec:80:c5:15
	ethaddr=00:10:ec:00:c5:15
	ver=U-Boot 2013.04-rc1-00289-g497746b-dirty (Mar 22 2013 - 12:50:25)

etc.

Signed-off-by: Wolfgang Denk <wd@denx.de>