cpu/esp8266/include/periph_cpu.h overrides the default definition of adc_res_t from periph/adc with a definition which contains only one resolution. This gives compilation errorss if an application uses other resolutions. According to the documentation, adc_sample should return -1 if the resolution is not supported. All other CPUs override adc_res_t either to add new resolutions or to mark resolutions as unsupported. But they all allow to use them at the interface. Therefore, esp8266 uses now the default definition of adc_res_t and returns -1 if a solution is used in adc_sample that is not supported.
