Wait for EEFC flash to complete operations before reseting

[Lohrer]

For an Arduino Due with EEFC flash, setting the boot flash bit doesn't complete before a reset is performed. This makes it impossible to get the SAM device out of bootloader mode. Adding a simple check that flash operations are done after committing them fixes the issue.

Note, to program my Due from Ubuntu also requires #78 to get the 1200 baud reset working.

[shumatech]

The paradigm in the Flash classes is to wait for completion before performing the next operation to better pipeline commands. If there is a dependency that all flash operations must be complete prior to execution of a reset, then that dependency should be handled at a higher level. For example, change bossa.c to:

if (config.reset) {
// Ensure flash operation are complete prior to reset
flash->ready();
device.reset();
}

You'll need to add the ready() method to the Flash base class and implement it in each concrete class.

[Lohrer]

Closing this PR in favor of #151 which implements the correct Flash paradigm, as suggested above by @shumatech.