Core: Improve code quality

This patch fixes the following things:
* avoid implicit casting by using matching types or casting
* avoid implicit casting between enum tfm_buffer_share_region_e and
uint32_t types by replacing the enumerated type with uint32_t and its
constants with preprocessor constants
* make explicit that the return value of a function is not used
* remove function return type if not needed
* replace preprocessor if condition by ifdef if only definition is
  checked
* replace enumerated types not used by preprocessor constants

Change-Id: Ib718937f89631cae2212de1fc748c48461b683d0
Signed-off-by: Hugues de Valon <hugues.devalon@arm.com>