How to pass preprocessor variable to rboot?

Hello,

We use slightly modified rboot and we would like to define preprocessor variable used in rboot.c during firmware building.

We have tried to modify rboot.mk and add CFLAGS += -DFOO=${FOO} and run mos tool as FOO=\"bar\" mos build ... but it says:

rboot.mk:63: warning: undefined variable 'FOO'

It would be even better if we could use build_vars for this.

Is it possible?