i2c: Ensure bus isn't BUSY before starting transaction

See Issue #114. In short, any tx attempts made while a STOP symbol is
being sent will be dropped by the I2C peripheral.
i2c_reg: Initial commit
Add comparator registers
Add VREF registers
Merge pull request #117 from bgamari/i2c

I2C fixes
Merge pull request #118 from bgamari/master

Add CMP and VREF
timeout: Ensure all past-due queued tasks are run
Merge branch 'master' of https://github.com/mchck/mchck
stdio: Ignore writes on files without file_ops