km/scripts: product env and auto-reset for ramfs

This patch adds the possibility in both debug and ramfs modes to
optionally load an env file from /tftpboot/$tftppath (this is ignored if
not present, so the change is backward compatible). This gives the debug
and ramfs scripts the possibility to set uboot environment variables
that were previously asked the users to manually set (nfs path in debug
and rootfs size in ramfs).

Signed-off-by: Holger Brunck <holger.brunck@ch.abb.com>
CC: Tom Rini <trini@konsulko.com>