This patch changes the build time behaviour when using deprecated API within
Trusted Firmware. Previously the use of deprecated APIs would only trigger a
build warning (which was always treated as a build error), when
WARN_DEPRECATED = 1. Now, the use of deprecated C declarations will always
trigger a build time warning. Whether this warning is treated as error or not
is determined by the build flag ERROR_DEPRECATED which is disabled by default.
When the build flag ERROR_DEPRECATED=1, the invocation of deprecated API or
inclusion of deprecated headers will result in a build error.

Also the deprecated context management helpers in context_mgmt.c are now
conditionally compiled depending on the value of ERROR_DEPRECATED flag
so that the APIs themselves do not result in a build error when the
ERROR_DEPRECATED flag is set.

NOTE: Build systems that use the macro WARN_DEPRECATED must migrate to
using ERROR_DEPRECATED, otherwise deprecated API usage will...