tests/periph_gpio: let build fail without GPIO
tests/periph_gpio_exti: let build fail w/o GPIO
make: introduce and use periph_pwm feature
tests/periph_pwm: let build fail w/o PWM
make: introduce and use periph_random feature
tests/periph_random: let build fail w/o RANDOM
make: introduce and use periph_rtt feature
tests/periph_rtt: let build fail w/o RTT
make: introduce and use periph_cpuid feature
tests/periph_cpuid: let test fail w/o CPUID_ID_LEN
Merge pull request #1822 from LudwigOrtmann/featuremania

make + tests: add more Makefile.features