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