baremetal: Change input for sfdisk

To prevent sfdisk stopping with "bad input", use a empty string
instead of "-" (and doing so requires using "," as a separator).

Fixes bug 1166112

Change-Id: I5e4cffaa326fbafd1583d55744f921a40b3ae76f
Merge "baremetal: Change input for sfdisk"
Merge remote-tracking branch 'm_src_repo/citrix-fixes' into build-temp