zephyr: Add support for automatically calculcating max sectors

Adds a feature that will calculate the maximum number of sectors
that are needed for a build. Can be disabled to revert back to
the old behaviour by disabling CONFIG_BOOT_MAX_IMG_SECTORS_AUTO

Signed-off-by: Jamie McCrae <jamie.mccrae@nordicsemi.no>
diff --git a/boot/zephyr/Kconfig b/boot/zephyr/Kconfig
index effedfb..bf772ca 100644
--- a/boot/zephyr/Kconfig
+++ b/boot/zephyr/Kconfig
@@ -380,9 +380,21 @@
 	  with the public key information will be written in a format expected by
 	  MCUboot.
 
+config BOOT_MAX_IMG_SECTORS_AUTO
+	bool "Calculate maximum sectors automatically"
+	default y
+	help
+	  If this option is enabled then the maximum number of supported sectors per image will
+	  be calculated automatically from the flash erase sizes and size of each partition for
+	  the first image.
+
+	  If this information is not available, or multiple images are used, then this option
+	  should be disabled and BOOT_MAX_IMG_SECTORS should be set instead
+
 config BOOT_MAX_IMG_SECTORS
 	int "Maximum number of sectors per image slot"
 	default 128
+	depends on !BOOT_MAX_IMG_SECTORS_AUTO
 	help
 	  This option controls the maximum number of sectors that each of
 	  the two image areas can contain. Smaller values reduce MCUboot's