Initial library implementation

[keithm-xmos]

• Modify to support byte length instead of word length reads
• Investigate dummy clocks and impact on maximum clock freq in various flash parts
• Productize calibration code, including how users store it along their application flash usage 
• Validation of various read modes bitrev/nibble swap if it can be done in the inner loop

[xmos-jmccarthy]

Completed as of 5e895b0f8108061d1f9dce27ead715788b1e98cb