configs, arch-arm: don't search for default DTB and kernel

Before this commit, there were default magic DTB and kernel filenames
for some platforms.

This was inelegant and error prone, as it refered to out-of-tree files,
and set defaults which users almost always want to customize with
explicit command line options.

One result of this is that a wrong exception could be thrown if --kernel
was given but not --machine-type, since the default machine type
VExpress_EMM had a default kernel, and the code would always search for
the default filename even though --kernel was given:

IOError: Can't find file 'vmlinux.aarch32.ll_20131205.0-gem5' on path.

The defaults existed only for older machine types, and not for the
usually recommended VExpress_GEM5_V1, which suggests that this
deprecation should not affect many users.

Change-Id: Ia49298304f658701ea0800bd79e08db404a655c3
Reviewed-on: https://gem5-review.googlesource.com/c/15898
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Jason Lowe-Power <jason@lowepower.com>