It is the role of boards based on 'cpu/fe310' to give the configuration
for the rtc/rtt.
The fe310/periph/rtc implementation depends on having periph/rtt configured
by the board so depends on the board 'providing' the periph_rtt feature
and declaring the required macros.
It should not simply depend of the 'periph_rtt' module as this does not
enforce having a configuration for the module in the board.
In practice, when compiling, it would result in undefined 'RTT' symbols,
instead of the build system detecting it.
