gen_msvs_proj: write boolean for Debug attribute

Replace =1 with =true for yasm tool element. This aids in upgrading
e.g., vs9 project files to vs10.
build/x86-msvs/yasm.xml generated during conversion will require the
Separator attribute to be removed for the build to complete
successfully.

Change-Id: If75c4f9a925529740048882003e9d766c5ac4f0c