More tweaks to _config.h, _kernel.h; smem tweaks.

Details:
- Moved kernel-related definitions form bl2_config.h to bl2_kernel.h.
- Replaced #define of _GNU_SOURCE with #define of _POSIX_C_SOURCE. This
  accomplishes the same thing (enabling posix_memalign()) without enabling
  all of the GNU extensions we don't need.
- Defined the size of the static memory pool in terms of MC, KC, and NC,
  as well as two new constants that determine how many MCxKC blocks and
  how many KCxNC blocks should be allocated (defined in bl2_config.h).
- In the case of static memory pool exhaustion, replaced the generic
  bl2_abort() with a specific error code call.

version

@@ -1 +1 @@
-0.0.1-4
+0.0.1-5