Refactor internal flash TDB bounds determination

Default the size to the larger of two sectors or 10 pages, so that the
  computation works better on devices with a low sector to page size ratio.
Reduce code duplication.

Author: Kyle Kearney

features/storage/kvstore/conf/filesystem/mbed_lib.json

@@ -2,11 +2,11 @@
     "name": "storage_filesystem",
     "config": {
         "rbp_internal_size": {
-            "help": "Default is the size of the 2 last sectors of internal flash",
+            "help": "Default is the larger of the last 2 sectors or last 10 pages of flash.",
             "value": "0"
         },
         "internal_base_address": {
-            "help": "If default, base address is the first sector after the application code",
+            "help": "If default, base address is set to internal_size bytes before the end of flash.",
             "value": "0"
         },
         "filesystem": {
@@ -36,7 +36,7 @@
     },
     "target_overrides": {
         "MCU_PSOC6": {
-            "rbp_internal_size": 7168
+            "rbp_internal_size": "7168"
         }
     }
 }