Update selection of startup file and linker script

As startup files and linker scripts are now separate targets, rename
the alias `mdh-platform-startup` to `mdh-platform-startup-linker` and
turn it into an interface library that links a platforms startup target
and linker script target, as it is not possible to alias two targets
into one name.

Signed-off-by: Lingkai Dong <lingkai.dong@arm.com>