Interface: Clean up the tfm_api.h header file

This file does not look like an "interface" file as it contains
some SPM internal usage definitions.

This patch:
- Moves the SPM internal definitions to spm.h.
- Use the error codes in `internal_status_code.h` instead of the
  ones in tfm_api.h.
- Removes error codes in tfm_api.h.
- Renames tfm_api.h to tfm_veneers.h to reflect what it does.
- Corresponding changes in source files.

Change-Id: Iba5d1aae6837c9f0bf52c9a0c456023d19a735ea
Signed-off-by: Kevin Peng <kevin.peng@arm.com>
diff --git a/secure_fw/partitions/lib/runtime/service_api.c b/secure_fw/partitions/lib/runtime/service_api.c
index 3e9576d..65bb844 100644
--- a/secure_fw/partitions/lib/runtime/service_api.c
+++ b/secure_fw/partitions/lib/runtime/service_api.c
@@ -12,9 +12,9 @@
 #include "utilities.h"
 
 __attribute__((naked))
-int32_t tfm_core_get_boot_data(uint8_t major_type,
-                               struct tfm_boot_data *boot_status,
-                               uint32_t len)
+psa_status_t tfm_core_get_boot_data(uint8_t major_type,
+                                    struct tfm_boot_data *boot_status,
+                                    uint32_t len)
 {
     __ASM volatile(
         "SVC    "M2S(TFM_SVC_GET_BOOT_DATA)"               \n"