tools: Add explicit clean rule for SeaBIOS

tools: Add explicit clean rule for SeaBIOS

Since seabios-dir is cloned during build we need to check that it exists before
recursing into it for clean, following the pattern used for qemu-*-dir etc.

Also remove usage of "buildmakevars2shellvars" except when used to poopulate
the environment for qemu-xen-traditional's xen-setup script, which is the only
user.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
Committed-by: Ian Jackson <ian.jackson@eu.citrix.com>