Makefile: Do not pass -E to mkimage when SPL has full fitImage support

When the SPL has full fitImage support, do not generate fitImage with
external data. The full fitImage code assumes the entire fitImage is
loaded in RAM, and the SPL uses fdt_totalsize() to determine the size
of the whole fitImage, which can not work with external data.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Tom Rini <trini@konsulko.com>