SAMA5: Add ADC-side of the logic to hook in timer/counter logic needed to drive  periodic ADC sampling

scanf() fixes from kfrolov: 1) sscanf() function hangs in the following example: sscanf(2, %u,%u,%u,%u, ...), 2) sscanf() returns incorrect number of parsed numbers if some arguments can't be parsed: sscanf(=2, %u,%u,%u,%u,...)==1 instead of 0, and 3) using of char* instead of const char* in vsscanf function leads to warnings from GCC.

SAMA5:  Hook RTC into build system; Finish RTC alarm logic; Verify correct behavior of the basic RTC functionality

Add instructions to enable RTC in the nsh/ configuration; RTC enabled by default in the demo configuration