Added spi file.
Updated header file for spi library.
Some improvements on spi library.
Implemented spi init function for KL15.
Implemented spi read and write functions.
Added utility.h file for useful functions and definitions.
Managed spi0 pins of KL15.
Small fix on spi number control.
Small fix on spi number control. (reverse-merged from commit 3ce0f22737d4c3854746cf59f7b01c7efaff1e4c)