Fix corruption of mcuboot-s1 + secondary/netcore partitions

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] https:/AudioStreamingPlatform/sdk-mcuboot/commit/766be5d72bbb722aea365036588b5027ff1e5400
[2] https:/nrfconnect/sdk-mcuboot/commit/7c3d7ed725919b5c0cce8688d565dec59957cb9b

boot: boot_serial: add custom BNO_IMAGE_TRAILER support

a0e8b6f boot: boot_serial: add custom BNO_IMAGE_TRAILER support
9457715 pm: add inequally sized primary and secondary partitions

boot: serial: "image list" enhanced to support mcuboot update

766be5d boot: serial: "image list" enhanced, client can program correct mcuboot_sX.bin
e6456ee boot: serial: unable to detect cbor encode overrun (wrong buf-size)
6c0480f boot/zephyr: increase mcuboot partition(s), was too small
9a74efb zephyr: prj: increase size of mcuboot partition
9cfd155 Corrected Mbed OS boot
82afb20 boot: add support for netcore serial updates