Replace flash partitioning terminology

This change replaces the slot 0/1 terminology with primary/secondary
slot and replaces FLASH_AREA_IMAGE_0/1 with
FLASH_AREA_IMAGE_PRIMARY/SECONDARY. This naming convention may be more
understandable, fits better to MCUs with multiple images and it is an
architecture agnostic alternative as well.

Change-Id: I655a585f6ae023852c671ee6635399efe25209c9
Signed-off-by: David Vincze <david.vincze@arm.com>
Signed-off-by: David Brown <david.brown@linaro.org>
diff --git a/samples/zephyr/overlay-skip-primary-slot-validate.conf b/samples/zephyr/overlay-skip-primary-slot-validate.conf
new file mode 100644
index 0000000..e94518e
--- /dev/null
+++ b/samples/zephyr/overlay-skip-primary-slot-validate.conf
@@ -0,0 +1,3 @@
+# Kconfig overlay for building without validating primary slot.
+
+# CONFIG_BOOT_VALIDATE_SLOT0 is not set