makefile: rm -f -> $(RM)

[marcin-laszewski]

Simple makefile upgrade: rm -f -> $(RM)

[ghaerr]

I'm not sure if this change actually helps, or instead possibly introduces more risk, into the ELKS build process. The make RM variable is not required to be defined by POSIX, and is not defined in our repo. It is entirely unused, except for I notice within the libc/ build. So a portion of the repo is already dependent on it, although that could be easily remedied by defining the RM variable in libc/Makefile.

On the other hand, GNU Make does define the variable by default to rm -f. For systems not running GNU make, the variable could be defined elkscmd/Makefile.defs, which would eliminate the risk for elkscmd/*/Makefiles.

Have you tested a full development system rebuild, and on which hosts? In particular I would be hesitant to change this in tools/Makefile. This change would also trigger half-hour automated CI rebuilds for the entire development toolchain.

[ghaerr]

Completed via #2028.