Build: Combine compile time MCUBoot switches

Remove multiple compile time MCUBoot switches and combine their
functionality into the 'MCUBOOT_UPGRADE_STRATEGY' switch.

Change-Id: Ibed68983ca0e2ed3c291d56514e8ad93ac0dbeba
Signed-off-by: David Vincze <david.vincze@arm.com>
13 files changed