Allow fw env tools to be available as library

Sometimes it can be useful to link the fw_ tools instead
of having the fw_setenv/fw_printenv installed.
Patch exports the tool as library and allowes to link it
with own programs.

Signed-off-by: Stefano Babic <sbabic@denx.de>
CC: Tom Rini <trini@konsulko.com>
Reviewed-by: Tom Rini <trini@konsulko.com>