plat/*: Add linker scripts to link dependencies

plat/*: Add linker scripts to link dependencies

This commit adds needed linker scripts as dependency to the linking rule
of the final image. This dependency was mostly indirectly covered via
libraries. However, updates to platform linker scripts did not trigger
relinking the final image.
Additionally, pure API libraries (like `lib/ukfalloc`) would not even cause
building a linker script that is shipped with the library or injected with
an `EACHOLIB_SRCS` declaration. The library buildrule is generated but
because no library object is generated, the linker script was never be part
of a dependency list.

Signed-off-by: Simon Kuenzer <simon@unikraft.io>
Reviewed-by: Delia Pavel <delia_maria.pavel@stud.acs.upb.ro>
Approved-by: Razvan Deaconescu <razvand@unikraft.io>
Tested-by: Unikraft CI <monkey@unikraft.io>
GitHub-Closes: #868