Move max image sector config to mcuboot_config.h

Take the opportunity to clean up a bit of platform cruft that has
gotten into bootutil by moving it to mcuboot_config.h, and ensuring it
is documented in the template config file.

Signed-off-by: Marti Bolivar <marti@opensourcefoundries.com>
diff --git a/boot/bootutil/src/bootutil_priv.h b/boot/bootutil/src/bootutil_priv.h
index d7ab258..b0dab08 100644
--- a/boot/bootutil/src/bootutil_priv.h
+++ b/boot/bootutil/src/bootutil_priv.h
@@ -91,15 +91,7 @@
     uint8_t image_ok;
 };
 
-#if defined(__BOOTSIM__)
-#define BOOT_MAX_IMG_SECTORS       128
-#elif defined(__ZEPHYR__)
-#define BOOT_MAX_IMG_SECTORS       CONFIG_BOOT_MAX_IMG_SECTORS
-#elif defined(MCUBOOT_MYNEWT)
-#define BOOT_MAX_IMG_SECTORS       MYNEWT_VAL(BOOTUTIL_MAX_IMG_SECTORS)
-#else
-#error "Invalid target OS"
-#endif
+#define BOOT_MAX_IMG_SECTORS       MCUBOOT_MAX_IMG_SECTORS
 
 /*
  * The current flashmap API does not check the amount of space allocated when
diff --git a/boot/mynewt/mcuboot_config/include/mcuboot_config/mcuboot_config.h b/boot/mynewt/mcuboot_config/include/mcuboot_config/mcuboot_config.h
index 4e169ab..f0842a3 100644
--- a/boot/mynewt/mcuboot_config/include/mcuboot_config/mcuboot_config.h
+++ b/boot/mynewt/mcuboot_config/include/mcuboot_config/mcuboot_config.h
@@ -52,4 +52,6 @@
 #define MCUBOOT_OVERWRITE_ONLY_FAST 1
 #endif
 
+#define MCUBOOT_MAX_IMG_SECTORS       MYNEWT_VAL(BOOTUTIL_MAX_IMG_SECTORS)
+
 #endif /* __MCUBOOT_CONFIG_H__ */
diff --git a/boot/zephyr/include/mcuboot_config/mcuboot_config.h b/boot/zephyr/include/mcuboot_config/mcuboot_config.h
index 7daa58f..0b65cd0 100644
--- a/boot/zephyr/include/mcuboot_config/mcuboot_config.h
+++ b/boot/zephyr/include/mcuboot_config/mcuboot_config.h
@@ -48,6 +48,8 @@
  */
 #define MCUBOOT_USE_FLASH_AREA_GET_SECTORS
 
+#define MCUBOOT_MAX_IMG_SECTORS       CONFIG_BOOT_MAX_IMG_SECTORS
+
 #endif /* !__BOOTSIM__ */
 
 #endif /* __MCUBOOT_CONFIG_H__ */