boot: zephyr: sysflash: restore FLASH_AREA_IMAGE_x macros

When introducing the MCUBOOT_MCUMGR_SIMPLE_IMAGE_INDEX option[1], the
following macros were changed:

        FLASH_AREA_IMAGE_PRIMARY(x)

        FLASH_AREA_IMAGE_SECONDARY(x)

These macros are used in other parts of the MCUboot codebase as well,
so it seems more appropriate to make the config implementation a bit
uglier to restore the intended behavior[2] of these macros.
The SIMPLE_IMAGE_INDEX is only relevant for `boot_serial.c` to simplify
the `mcumgr` image numbering.

Note: one case is unreachable and commented out since the SCRATCH
      partition definition also is not available here.

[1] 766be5d
[2] nrfconnect@7c3d7ed

Author: Emil-Juhl

boot/boot_serial/src/boot_serial.c

@@ -319,7 +319,33 @@ static int flash_area_id_from_upload_id(uint32_t upload_id)
     if (upload_id >= 1) {
         uint32_t image_index = (upload_id - 1) / 2;
         uint32_t slot = (upload_id - 1) % 2;
-        return flash_area_id_from_multi_image_slot(image_index, slot);
+        /* This switch is essentially a manual inlining of the
+         * `flash_area_id_from_multi_image_slot` function.
+         * The return value has been tweaked to fit the SIMPLE_IMAGE_INDEX
+         * scheme though.
+         * This solution may be preferred over tweaking the FLASH_AREA_IMAGE_x
+         * macros as they are used in other places as well.
+         * */
+        switch(slot) {
+        case 0:
+            switch(image_index) {
+            case 0: return PM_MCUBOOT_PRIMARY_ID;
+            case 1: return PM_S0_ID;
+            default: return 255;
+            }
+        case 1:
+            switch(image_index) {
+            case 0: return PM_MCUBOOT_SECONDARY_ID;
+            case 1: return PM_S1_ID;
+            default: return 255;
+            }
+    /* Case unreachable. Included for completeness
+     *  case 2:
+     *  return FLASH_AREA_IMAGE_SCRATCH;
+     * */
+        default:
+            return -1;
+        }
     }
     return -1;
 #elif defined(MCUBOOT_SERIAL_DIRECT_IMAGE_UPLOAD)

boot/zephyr/include/sysflash/sysflash.h

@@ -36,15 +36,15 @@ extern uint32_t _image_1_primary_slot_id[];
         ((x == 0) ?                            \
            PM_MCUBOOT_PRIMARY_ID :             \
          (x == 1) ?                            \
-          PM_S0_ID : \
+          PM_MCUBOOT_PRIMARY_ID : \
            255 )
 #endif
 
 #define FLASH_AREA_IMAGE_SECONDARY(x) \
         ((x == 0) ?                   \
             PM_MCUBOOT_SECONDARY_ID:  \
         (x == 1) ?                    \
-           PM_S1_ID :  \
+           PM_MCUBOOT_SECONDARY_ID :  \
            255 )
 #else