boot : Adding encrypted ram-load support

This patch adds the possibility to boot using ram-load with an
encrypted image. This is enabled when both the flags MCUBOOT_RAM_LOAD
and MCUBOOT_ENC_IMAGES are defined.

Signed-off-by: Fabio Utzig <utzig@apache.org>
Signed-off-by: Hugo L'Hostis <hugo.lhostis@arm.com>
Signed-off-by: Salome Thirot <salome.thirot@arm.com>
Change-Id: I7756c2c634d90a2e726117d6cfc5650653cf1b51