Replacing usage will come in further commits/PRs.

Unittests add libraries in 'BASELIBS' which do not have any rules to be built as
they are built by 'application.inc.mk', packages and the DIRS variable.
So make complains about missing target for the unittests archives.

The fix tells these files are generated when building '$(APPLICATION_MODULE).a'.

The bug was introduced by #8844

Fixes #8910

The FEATURES_CONFLICT check should be done on used features to also
test for included optional features.

Replace HEXFILE rule by a rule for each type.
HACK: Adding '-Oformat' after OFLAGS overwrite the value in flags.

* Add ELFFILE, HEXFILE, print-size targets

$(BINDIR)/$(APPLICATION_MODULE).a target builds libraries in RIOT and in DIRS.

Uses a FORCE phony target dependency to force rebuilding but still let
`make` use the file modification timestamps

Refactor osx-native linker rule into `_LINK` too.

.DEFAULT_GOAL was reset many times which removed 'all' from being the default
goal.
By chance it was then set to `link` so was working by some magic.

Refactor the code checking if command is present.
Message is taken from the compiler check as it was more precise than the others.

This variable can be used to specify the linker which should be used to
link C++ object files. This is needed in order to use g++ for linking
with TOOLCHAIN=llvm.

Set the application module name with APPLICATION_MODULE.
The default module name is "application_$(APPLICATION)".

This should fix name conflicts where an application/test has the same name as
an existing module and so both write to the same archive.

Seem to cause internal linker errors and linker segfaults under some
circumstances when using gold as the linker.