Previously the build process removed everything and did all the build
again on each make invocation.
This fixes this behaviour with two changes.
First dynamically find the list of files to build using find instead of
a manually written list.
Then use implicit rules to only build files that need to be built again
instead of recompiling everything.
Instead of hardcoding the names set them using a variable.
Also rename the target build to the name of the file it builds which is
in the ELFFILE variable.
