Adds support for nrf5340-DK board (only application core)

[rkudaj]

Integrates bootloader with nRF5430 application core. Adds two segger projects:

• bootloader working with nRF5340-DK board
• small app which replaces old mbr.hex (not available for nRF5340 in Nordic sdk)

In app_timer removes software interrupt, as this interrupt is not available on nRF5430 application core.

[rkudaj]

Tests for nrf52 were not done yet, I m planning to do them shortly when I get board with nrf52.

[rkudaj]

Tested on nrf52840 DK (_nosd) and nrf5340 DK boards. nrfx updated to newest release.

[rkudaj]

I updated this pr to a newer one, so closing this one.

[rkudaj]

I updated this pr to a newer one, so closing this one.