added optimization menu option for M0

[hathach]

tested with circuit playground express with msc_external_flash.ino examples. The platforms.txt already support it, we only need to update boards.txt

Small Os
    Sketch uses 35664 bytes (13%) of program storage space. Maximum is 262144 bytes.
Fast O2
   Sketch uses 36256 bytes (13%) of program storage space. Maximum is 262144 bytes
Faster O3
    Sketch uses 42876 bytes (16%) of program storage space. Maximum is 262144 bytes.
Fastest Ofast
   Sketch uses 42876 bytes (16%) of program storage space. Maximum is 262144 bytes
Here be dragons  -Ofast -funroll-loops
   Sketch uses 47928 bytes (18%) of program storage space. Maximum is 262144 bytes.

[ladyada]

thank you :)