Fix to flash API on EFM32

* Since mbed does not overwrite itself, make the flashing routines run out of flash by default
* Report a writeable size of 4 bytes (previously erroneously reported a full eraseable page as the minimum write size)

Author: stevew817

targets/TARGET_Silicon_Labs/TARGET_EFM32/flash_api.c

@@ -112,14 +112,14 @@ uint32_t flash_get_sector_size(const flash_t *obj, uint32_t address)
 
 /** Get page size
  *
+ * The page size defines the writable page size
  * @param obj The flash object
- * @param address The page starting address
  * @return The size of a page
  */
 uint32_t flash_get_page_size(const flash_t *obj)
 {
     (void)obj;
-    return FLASH_PAGE_SIZE;
+    return sizeof(uint32_t);
 }
 
 /** Get start address for the flash region

targets/targets.json

@@ -2820,7 +2820,7 @@
     "EFM32": {
         "inherits": ["Target"],
         "extra_labels": ["Silicon_Labs", "EFM32"],
-        "macros": ["MBEDTLS_CONFIG_HW_SUPPORT", "MBED_TICKLESS"],
+        "macros": ["MBEDTLS_CONFIG_HW_SUPPORT", "MBED_TICKLESS", "EM_MSC_RUN_FROM_FLASH"],
         "public": false
     },
     "EFM32GG990F1024": {