doc: Rework PORTING.md to reflect changes to flash_area API

## Flash Map

The bootloader requires a `flash_map` to be able to know how the flash is

partitioned. A `flash_map` consists of `struct flash_area` entries

specifying the partitions, where a `flash_area` defined as follows:

The bootloader requires to be able to address flash regions where the code

for mcuboot and images of applications are stored, in system-agnostic way.

For that purpose the mcuboot uses ID, which is integer (uint8_t) number

that should uniquely identify each flash region.

Such flash regions are served by object of `const struct flash_area` type while

layout of these objects is gathered under `flash_map`.

The common code of mcuboot, that is non-system specific, does not directly

access contents of that object and never modifies it, instead it calls

`flash_area_` API to perform any actions on that object.

This way systems are free to implement internal logic of flash map or define

`struct flash_area` as they wish; the only restriction is that ID should be

uniquely tied to region characterized by device, offset and size.

Changes to common mcuboot code should not affect system specific internals

of flash map, on the other side system specific code, within mcuboot, is

is not restricted from directly accessing `struct flash_area` elements.

An implementation of `struct flash_area` may take form of:

```c

struct flash_area {

    uint8_t  fa_id;         /** The slot/scratch identification */

    uint32_t fa_off;        /** The flash offset from the beginning */

    uint32_t fa_size;       /** The size of this sector */

};

```

The above example of structure hold all information that is currently required

by mcuboot, although the mcuboot will not be trying to access them directly,

instead a system is required to provide following mandatory getter functions:

```c

/*< Obtains ID of the flash area characterized by `fa` */

int     flash_area_get_id(const struct flash_area *fa);

/*< Obtains ID of a device the flash area `fa` described region resides on */

int     flash_area_get_device_id(const struct flash_area *fa)

/*< Obtains offset, from the beginning of a device, the flash area described

 * region starts at */

uint32_t flash_area_get_off(const struct flash_area *fa)

/*< Obtains size, from the offset, of the flash area `fa` characterized region */

uint32_t flash_area_get_size(const struct flash_area *fa)

```

`fa_id` is can be one of the following options:

The mcuboot common code uses following defines that should be defined by system

specific header files and are used to identify destination of flash area by ID:

```c

/* Independent from multiple image boot */

#define FLASH_AREA_IMAGE_SECONDARY    6

```

The functions that must be defined for working with the `flash_area`s are:

The numbers, given above, are provided as an example and depend on system

implementation.

The main, also required, set of API functions that perform operations on

flash characterized by `struct flash_area` objects is as follows:

```c

/*< Opens the area for use. id is one of the `fa_id`s */