warp7: hab: Set environment variable indicating IVT offset

This patch introduces the environment variable ivt_offset. When we define a
load address for Linux or DTB or any file the IVT associated with that file
is prepended. We extract the actual load addresses from u-boot.cfg and feed
these values into the code-signing process - hence we want u-boot to have
the real load addresses exported in uboot.cfg.

ivt_offset represents the addition or subtraction from the load address
that must happen to find an IVT header.

Signed-off-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>
Tested-by: Breno Lima <breno.lima@nxp.com>
Reviewed-by: Fabio Estevam <fabio.estevam@nxp.com>