Cypress: Add entering deep sleep in MCUBootApp, code improvements

- improved code in MCUBootApp
- added possibility to enter deep sleep mode after mcuboot app execution
- minor refactoring

Signed-off-by: dmiv <dmiv@cypress.com>
Signed-off-by: Roman Okhrimenko <roman.okhrimenko@cypress.com>
Signed-off-by: Taras Boretskyy <taras.boretskyy@cypress.com>