Prepare BSP for Internal Flash filesystem

[hathach]

This PR prepare BSP for fatfs support on Internal Flash of samd21 Adafruit_InternalFlash . The board under test is ProxLight Trinkey which does not have spi flash. Changes include

• change bossac default = 1.7 to bossac18 since 1.7 does a full chip erase every upload which destroy the filesystem. So far everything seems to work well
• fake the SPI interface with dummy pin (SPI_INTERFACES_COUNT change to 1). This is because SdFat requires existence of SPI object to compile. Even though we use it as generic fatfs with block device API.

@ladyada please let me know if faking SPI make sense to you, otherwise, I will try to figure out another way to go around SdFat compilation.

[ladyada]

oh this is annoying.... yeah sure, its ok!