Add new target option to print/show the build size.

We should discuss if this is something we'd like to add, and if this is the right format to enable it (with an entry in the target.json file).

The size info is added by using the arm-none-eabi-size tool and the -Wl,--print-memory-usage linker flag.

With this change, and by setting the "show_size": true option in target.json, we get this additional info printed during builds:

$ python build.py

...

[ 99%] Linking CXX executable MICROBIT
Memory region         Used Size  Region Size  %age Used
             MBR:        2816 B         4 KB     68.75%
              SD:        105 KB       108 KB     97.22%
           FLASH:      127888 B       364 KB     34.31%
      BOOTLOADER:       25580 B        28 KB     89.22%
        SETTINGS:        4899 B         8 KB     59.80%
            UICR:           8 B          8 B    100.00%
          NOINIT:          16 B         16 B    100.00%
             RAM:      120768 B     122816 B     98.33%
Print total size info:
   text    data     bss     dec     hex filename
 239556  140823    8992  389371   5f0fb /Users/microbit-carlos/workspace/mbef/codal/microbit-v2-samples/build/MICROBIT
[ 99%] Built target MICROBIT
[100%] converting to bin file.
[100%] converting to hex file.
[100%] Built target MICROBIT_hex
[100%] Built target MICROBIT_bin

lancaster-university / microbit-v2-samples

Add new target option to print/show the build size. #81