servoflash.py: switch to futility + cleanups

By default, use futility instead of flashrom for read/write operations, as futility ensures that sections with PRESERVE flags, such as VPD and HWID in the GBB, are maintained.

Add some minor refinements and cleanups to servoflash.py code.

Signed-off-by: Laura Nao laura.nao@collabora.com