Build: Refinements for tfm_log CMake library

This patch:
- merges the tfm_log_interface and tfm_log libraries.
  tfm_log_interface was used only to expose an include directory.
  Targets need that directory now links tfm_log directly.
- changes the tfm_log library from INTERFACE to STATIC.
  tfm_log was linked by many other static libraries.
  This caused the source file of it (tfm_log_raw.c) being compiled for
  multiple times as it is an INTERFACE library.
  Changing to STATIC can avoid this issue.

Change-Id: Idac0a03c180aa9d5dd66b4ea425413435f5c765a
Signed-off-by: Kevin Peng <kevin.peng@arm.com>
2 files changed