travis-ci: centralize ~/.buildman editing

Any time an x86 toolchain is used, we need to edit ~/.buildman to
reference it. Move the editing logic into a central place so that it
doesn't have to be duplicated everywhere that uses the x86 toolchain;
future patches will add additional cases where it's used.

It would be nice if we could unconditionally write all of ~/.buildman at
once. Unfortunately, buildman fails if any toolchain mentioned in a
toolchain-prefix entry doesn't exist, even if it doesn't need to use it
for the current build.

The sandbox/x86 build definition currently does nothing more than edit
~/.buildman; no builds are run. Fix this by not defining a custom script
for this build, and hence preventing that stanza from replacing the
default script.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Reviewed-by: Heiko Schocher <hs@denx.de>