cmd_bootm.c: Add 'booti' for ARM64 Linux kernel Images

cmd_bootm.c: Add 'booti' for ARM64 Linux kernel Images

The default format for arm64 Linux kernels is the "Image" format,
described in Documentation/arm64/booting.txt.  This, along with an
optional gzip compression on top is all that is generated by default.
The Image format has a magic number within the header for verification,
a text_offset where the Image must be run from, an image_size that
includes the BSS and reserved fields.

This does not support automatic detection of a gzip compressed image.

Signed-off-by: Tom Rini <trini@ti.com>