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>
diff --git a/log/CMakeLists.txt b/log/CMakeLists.txt
index 9860934..e25920d 100644
--- a/log/CMakeLists.txt
+++ b/log/CMakeLists.txt
@@ -6,21 +6,19 @@
 #-------------------------------------------------------------------------------
 
 ############################# Ns Log ###########################################
-add_library(tfm_log INTERFACE)
-add_library(tfm_log_interface INTERFACE)
+add_library(tfm_log STATIC)
 
 target_sources(tfm_log
-    INTERFACE
+    PRIVATE
         tfm_log_raw.c
 )
 
-target_include_directories(tfm_log_interface
-    INTERFACE
+target_include_directories(tfm_log
+    PUBLIC
         .
 )
 
 target_link_libraries(tfm_log
-    INTERFACE
-        tfm_log_interface
+    PRIVATE
         platform_common_interface
 )