boot: Change TLV tag to 16 bits

The current TLV tag is an unsigned 8-bit integer, that is stored with 8
bits of padding.  As the TLV tag is defined to be little endian
(although the code doesn't properly handle this), we can use the 8 bits
of padding as the upper 8-bits, treating the TLV tag as a 16 bit value,
and all existing tags will operate as they did before.

Change the types used throughout the code to represent the TLV to a
`uint16_t`.  Change the ANY tag type to `0xffff` instead of `0xff`.
This value is never stored, but will avoid conflicts with any future
allocated tags.

Signed-off-by: David Brown <david.brown@linaro.org>