"env grep" - add options to grep in name, value, or both.

Add options to "env grep" command:

-n : search only the envrironment variable names
-v : search only their values
-b : search both names and values (= default)

An option "--" will stop parsing options, so to print variables that
contain the striing "- " please use:

	env grep -- "- "

Or to print all environment varioables which have a '-' in their name,
use:

	env grep -n -- -

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