Build: Check MCUboot strategy configuration

Validate the value of the MCUBOOT_UPGRADE_STRATEGY variable and
set the accepted values for its cache entry.
It helps to avoid misconfiguration which can remain hidden.

Remove unused UPGRADE_STRATEGY string variable and unnecessary check.

Change-Id: I6b8529dafe51cbaf2b26a5ac56a710c9687812ae
Signed-off-by: David Vincze <david.vincze@linaro.org>
3 files changed