tools: replace private PREFIX with automake prefix

tools: replace private PREFIX with automake prefix

Reuse automake variable instead of inventing a private variable.

Only qemu-traditional expects PREFIX in environment, which is provided
by buildmakevars2shellvars.

Signed-off-by: Olaf Hering <olaf@aepfle.de>
Cc: Ian Jackson <ian.jackson@eu.citrix.com>
Cc: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Cc: Ian Campbell <ian.campbell@citrix.com>
Cc: Wei Liu <wei.liu2@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>