Generate a .pe script for one design size at a time.

Generate a .pe script for one design size at a time.

In make

 a b: c
   command

is shorthand for

 a: c
   command
 b: c
   command

this means that in parallel builds the same command may be invoked twice.

Fix this by associating each command with only one target, and adding
dummy dependencies. Also, generate the .pe for only one design size at
a time.