Replace in memap CSV report fields

Changes:
* This change impacts #2047
* In code summary columns were shortened and names changed a bit,
  see [here](https://github.com/mbedmicro/mbed/pull/2047/files?w=1#diff-f9cb941bde5647a5763e18fc220efc51R410)
* There is a need to allign memap CSV report fields (summary columns) with what
  is in code now. This discrapency may create problems when generating reports
  in the future. All columns should have the same name.
* memap CSV sumamry fields names changes:
  'total_static_ram' -> 'static_ram'
  'allocated_heap' -> 'heap'
  'allocated_stack' -> 'stack'

See example of discrepancy:

'total_static_ram' vs 'static_ram'
'allocated_heap' vs 'heap'
'allocated_stack' vs 'stack'

What is in code and presented with prettytable:

+----------------------------------------+--------+-----------+------------+-------+-------+-----------+-------------+
| name                                   | target | toolchain | static_ram | stack |  heap | total_ram | total_flash |
+----------------------------------------+--------+-----------+------------+-------+-------+-----------+-------------+
| tests-mbed_drivers-c_strings           | K64F   | GCC_ARM   |      11468 | 32768 | 65540 |    109776 |       74182 |
| tests-mbed_drivers-callback            | K64F   | GCC_ARM   |      11980 | 32768 | 65540 |    110288 |       88406 |
| tests-mbedmicro-rtos-mbed-threads      | K64F   | GCC_ARM   |      11468 | 32768 | 65540 |    109776 |       75918 |
| tests-mbedmicro-rtos-mbed-timer        | K64F   | GCC_ARM   |      10788 | 32768 | 65540 |    109096 |       68534 |
+----------------------------------------+--------+-----------+------------+-------+-------+-----------+-------------+

vs CSV:

```
total_static_ram,allocated_heap,allocated_stack,total_ram,total_flash
10788,65540,32768,109096,67486
```

vs memstats_map.JSON:

```json
[
    {
        "module": "Fill",
        "size": {
            ".data": 4,
            ".bss": 2257,
            ".text": 173
        }
    },

    {
        "summary": {
            "total_flash": 67486,
            "allocated_stack": 32768,
            "total_static_ram": 10788,
            "total_ram": 109096,
            "allocated_heap": 65540
        }
    }
]
```

Author: PrzemekWirkus

tools/memap.py

@@ -436,16 +436,16 @@ def generate_output(self, export_format, file_output=None):
                     csv_module_section += [i+k]
                     csv_sizes += [self.modules[i][k]]
 
-            csv_module_section += ['total_static_ram']
+            csv_module_section += ['static_ram']
             csv_sizes += [subtotal['.data']+subtotal['.bss']]
 
-            csv_module_section += ['allocated_heap']
+            csv_module_section += ['heap']
             if subtotal['.heap'] == 0:
                 csv_sizes += ['unknown']
             else:
                 csv_sizes += [subtotal['.heap']]
 
-            csv_module_section += ['allocated_stack']
+            csv_module_section += ['stack']
             if subtotal['.stack'] == 0:
                 csv_sizes += ['unknown']
             else: