env: allow to export only selected variables

New syntax:
	env export [-t | -b | -c] [-s size] addr [var ...]

With this change it is possible to provide a list of variables names
that shall be exported.  Whenno arguments are given, the whole
environment gets exported.

NOTE: The new handling of the "size" argument means a change to the
user API.

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