boot: Unify the interpretation of TLV entry fields

Modify the definition of certain shared data TLV entry header fields to
be aligned with the image TLV's behaviour (the meaning of the tlv_len
field was different) and achieve unified interpretation.

Modify some macro definitions that use bitwise shift operators to
prevent certain errors from occurring due to implementation-defined
behaviour.

Change-Id: Ib736703a391d390a87d629a8ac4e77e1789120a5
Signed-off-by: David Vincze <david.vincze@linaro.org>