build: Use filechk rules to create and update u-boot.lds

Prior to this patch, the top-level linker script u-boot.lds
used a simple $(call if_changed) check when generated.
That mechanism misses cases where a possible include file
change induces a change in the u-boot.lds too.

This patch converts it to a stronger check using ($call filechk)
that will also notice differences in file contents and
will catch changes due to pre-processing as well.

Signed-off-by: Jon Loeliger <jon.loeliger@oracle.com>