boot: Fix IS_ENCRYPTED macro definition

The previous definition did not work as setting any type of flag would
make IS_ENCRYPTED true.

Signed-off-by: Salome Thirot <salome.thirot@arm.com>
(cherry picked from commit 23862672)
Signed-off-by: Andrzej Puzdrowski <andrzej.puzdrowski@nordicsemi.no>