Add config target.internal-flash-uniform-sectors and set it to false for STM32L2/4/7

Some internal flashes have non-uniform sectors, and for those
ones we want to skip the initialization of default LittleFileSystem
on FlashIAPBlockDevice (unless the user specifies an address
range that's uniform).

This commit adds a config to indicate if sectors are uniform.

Author: LDong-Arm

targets/targets.json

@@ -84,6 +84,10 @@
             "default-adc-vref": {
                 "help": "Default reference voltage for ADC (float)",
                 "value": "NAN"
+            },
+            "internal-flash-uniform-sectors": {
+                "help": "Target's internal flash has uniform sector sizes",
+                "value": true
             }
         }
     },
@@ -1344,6 +1348,9 @@
                 "macro_name": "CLOCK_SOURCE"
             }
         },
+        "overrides": {
+            "internal-flash-uniform-sectors": false
+        },
         "device_has_add": [
             "ANALOGOUT",
             "CAN",
@@ -1495,6 +1502,9 @@
                 "macro_name": "CLOCK_SOURCE"
             }
         },
+        "overrides": {
+            "internal-flash-uniform-sectors": false
+        },
         "device_has_add": [
             "SERIAL_ASYNCH",
             "FLASH",
@@ -2109,7 +2119,8 @@
             }
         },
         "overrides": {
-            "lpticker_delay_ticks": 0
+            "lpticker_delay_ticks": 0,
+            "internal-flash-uniform-sectors": false
         },
         "macros_add": [
             "MBED_TICKLESS",