Make the build system notice configuration file changes

[Nicolas Pouillon]

When makeing two kernels with different configuration files in a row, the build system does strange mix for the second one. This is because the dependency tracking is done only on the time of files, and not path.

We (I) should add a dependency tracking on the contents, or the name of the file...

[Joël Porquet]

The build-system is not able to track symbolic link for configuration file either.

• I build a kernel with a config file being actually a link to a config file
• I overwrite this link with another one, linking to a different config file (ln -sf ...)
• the new build system does not "see" the new config file
• if I "touch" the new link, the build system recompile everything, as expected

[Nicolas Pouillon]

Fixed in [1054]