Right now the gain and reference are hard coded. Until it is configurable
at runtime prefer the same default as with Arduino Core. That is,
GAIN_DIV2 and REFSEL_INTVCC1. This way we can properly measure the
battery voltage.
This change is for all boards with a sam0 cpu. This cpu just has one ADC.
It is unnecessary to have defines with ADC_0_ prefix as if multiple ADCs
are possible.
Some defines were not used, such as ADC_0_EN, ADC_0_CHANNELS,
ADC_MAX_CHANNELS, ADC_0_CLK_SOURCE, ADC_0_CHANNELS
Change all ADC_0_ prefixes to ADC_
cpu/$(CPU)/Makefile.features and cpu/$(CPU)/Makefile.dep are
automatically included
Part of moving CPU/CPU_MODEL definition to Makefile.features to have it
available before Makefile.include.
PORT_LINUX and PORT_DARWIN are evaluated by
`makefiles/tools/serial.inc.mk` and some boards Makefile.include.
Their value does not need to be exported.
This also globally removes the line saying that the exports are needed
for flash rule.
The currently supported SAM0 MCUs (samd21, saml21, saml1x) share the same
Timer peripheral, yet each of them carries it's own copy of the Timer
driver.
This introduces a new timer driver that is common for all sam0 MCUs and
uses structs for configuration instead of defines.
