boot: zephyr: Add Cortex-R support

Add Cortex-R support by using ifdefs to change the vector table, remove
CORTEX_M only code and do an explicit bx instruction to switch from
Thumb to ARM mode on exit of the MCUboot application.

Signed-off-by: Mika Braunschweig <mika.braunschweig@siemens.com>