Increase slot size to all available internal flash space

Author: pennam

mbed_app.json

@@ -57,7 +57,7 @@
             "fat_chan.ff_use_mkfs": 0,
             "fat_chan.ff_fs_rpath": 0,
             "mcuboot.primary-slot-address": "0x8020000",
-            "mcuboot.slot-size": "0xC0000",
+            "mcuboot.slot-size": "0x1E0000",
             "mcuboot.scratch-address": "0x80E0000",
             "mcuboot.scratch-size": "0x20000",
             "mcuboot.max-img-sectors": "0x180",