Conditional syscfg.yml restrictions

[Kevin Townsend]

I'm trying to add I2C support to the existing (SPI only) BME280 driver, 
but I'm not sure how to enforce the following conditional restrictions 
in the syscfg.yml file(s):

 * If BME280_SHELL_ITF_TYPE == 1, I2C_0 must be set
 * If BME280_SHELL_ITF_TYPE == 0, SPI_0_MASTER must be set
How would you check the contents of a field, rather than simply testing 
'on' or 'off', and verify the appropriate transport is enabled?