sysflash: Correct ALL_AVAILABLE_SLOTS when MCUBOOT is not image 2

Consider an nRF5340 application with the following sysbuild config:

```
SB_CONFIG_BOOTLOADER_MCUBOOT=y
SB_CONFIG_BOOT_SIGNATURE_TYPE_ECDSA_P256=y

SB_CONFIG_PM_EXTERNAL_FLASH_MCUBOOT_SECONDARY=y

SB_CONFIG_SECURE_BOOT_APPCORE=y
SB_CONFIG_SECURE_BOOT_NETCORE=y

SB_CONFIG_NETCORE_APP_UPDATE=y
SB_CONFIG_MCUBOOT_NRF53_MULTI_IMAGE_UPDATE=y
SB_CONFIG_MCUBOOT_MODE_OVERWRITE_ONLY=y
```

In this case, we have a total of 3 updatable images (app, net, mcuboot).

`MCUBOOT_IMAGE_NUMBER = 2` (i.e. `CONFIG_UPDATEABLE_IMAGE_NUMBER = 2`),
as the app secondary partition is reused for MCUBoot secondary.

`CONFIG_MCUBOOT_MCUBOOT_IMAGE_NUMBER = 2` as the net core image has
index 1.

In this case, we should fall through to the definition of
`ALL_AVAILABLE_SLOTS` that caters for 2 updatable images.

Author: ball-hayden

boot/zephyr/include/sysflash/pm_sysflash.h

@@ -31,12 +31,12 @@
 #define MCUBOOT_S0_S1_SLOTS
 #endif
 
-#if (MCUBOOT_IMAGE_NUMBER == 1) || (MCUBOOT_IMAGE_NUMBER == 2 && CONFIG_MCUBOOT_MCUBOOT_IMAGE_NUMBER != -1)
+#if (MCUBOOT_IMAGE_NUMBER == 1) || (MCUBOOT_IMAGE_NUMBER == 2 && CONFIG_MCUBOOT_MCUBOOT_IMAGE_NUMBER == 1)
 #define ALL_AVAILABLE_SLOTS FLASH_AREA_IMAGE_0_SLOTS
-#elif (MCUBOOT_IMAGE_NUMBER == 2) || (MCUBOOT_IMAGE_NUMBER == 3 && CONFIG_MCUBOOT_MCUBOOT_IMAGE_NUMBER != -1)
+#elif (MCUBOOT_IMAGE_NUMBER == 2) || (MCUBOOT_IMAGE_NUMBER == 3 && CONFIG_MCUBOOT_MCUBOOT_IMAGE_NUMBER == 2)
 #define ALL_AVAILABLE_SLOTS FLASH_AREA_IMAGE_0_SLOTS \
                             FLASH_AREA_IMAGE_1_SLOTS
-#elif (MCUBOOT_IMAGE_NUMBER == 3) || (MCUBOOT_IMAGE_NUMBER == 4 && CONFIG_MCUBOOT_MCUBOOT_IMAGE_NUMBER != -1)
+#elif (MCUBOOT_IMAGE_NUMBER == 3) || (MCUBOOT_IMAGE_NUMBER == 4 && CONFIG_MCUBOOT_MCUBOOT_IMAGE_NUMBER == 3)
 #define ALL_AVAILABLE_SLOTS FLASH_AREA_IMAGE_0_SLOTS \
                             FLASH_AREA_IMAGE_1_SLOTS \
                             FLASH_AREA_IMAGE_2_SLOTS