mk: fix parallel build of test resources

mk: fix parallel build of test resources

The build was failing sometimes when building with multiple
parallel jobs:
    # rm build/build/app/test/*res*
    # make -j6
    objcopy: 'resource.tmp': No such file

The reason is that each resource was built from the same temporary file.
The failure is seen because of a race condition when removing the
temporary file after each resource creation.
It also means that some resources may be created from the wrong source.

The fix is to have a different input file for each resource.
The source file is not directly used because it may have a long path
which is used by objcopy to name the symbols after some transformations.
When linking a tar resource, the input file is already in the current
directory. The hard case is for simply linked resources.
The trick is to create a symbolic link of the source file if it is not
already in the current build directory.
Then there is a replacement of dot by an underscore to predict the
symbol names computed by objcopy which must be redefined.

There is an additional change for the test_resource_c which is both
a real source file and a test resource. An intermediate file
test_resource.res is created to avoid compiling resource.c from the
wrong directory through a symbolic link.

Fixes: 1e9e0a6270 ("app/test: fix resource creation with objcopy on FreeBSD")

Signed-off-by: Thomas Monjalon <thomas.monjalon@6wind.com>
Acked-by: Olivier Matz <olivier.matz@6wind.com>