Revert "Revert "disk_util: use FAT32 on ESP""
Revert "Revert "disk_util: use FAT32 on ESP""

FAT32 seemed to aggravate https://github.com/coreos/bugs/issues/2284, but
now that that issue has been addressed, we can return to the correct
filesystem type.

This reverts commit 299f8fb3d10f00b46990bc582b2ba950d8408f11.

coreos/scripts,coreos/scripts,glevand/coreos--scripts,glevand/coreos--scripts