tup build: simplify project configuration variables
tup build: simplify project configuration variables

Merge CXXWARNINGS, CWARNINGS, CXXSTD, CSTD and DBGFLAGS with ASFLAGS,
CFLAGS, CXXFLAGS and LDFLAGS.
jasmin-j/distortos,jasmin-j/distortos,DISTORTEC/distortos,CezaryGapinski/distortos,DISTORTEC/distortos,CezaryGapinski/distortos,jasmin-j/distortos,DISTORTEC/distortos,CezaryGapinski/distortos,jasmin-j/distortos,CezaryGapinski/distortos,DISTORTEC/distortos,jasmin-j/distortos,CezaryGapinski/distortos