environment: ti: Add get_fit_config command to get FIT config string

When OE is packaging a dtb file into the FIT image it names the node based
on the dtb filename. Node names can't have "/" so it is turned into "_".
We select our FIT config using the "fdtfile" env var so we don't duplicate
the board_name to fdt logic. Result is fdtfile needs mangled when used to
select a config node from OE made FIT image. Do this here.

Signed-off-by: Andrew Davis <afd@ti.com>
Reviewed-by: Tom Rini <trini@konsulko.com>