refactor edk2 BuildCommand into slice of strings

[AtomicFS]

I think it would make it much nicer if the BuildCommand were a slice, the commands would then be much easier to read. Unfortunately this is a breaking change, but not that significant, people should be able to just change the JSON:

- "build_command": "source ./edksetup.sh; build -a X64 -p UefiPayloadPkg/UefiPayloadPkg.dsc -b DEBUG -t ${GCC_TOOLCHAIN_VERSION}",
+ "build_command": ["source ./edksetup.sh; build -a X64 -p UefiPayloadPkg/UefiPayloadPkg.dsc -b DEBUG -t ${GCC_TOOLCHAIN_VERSION}"],

That is if they do not want to take advantage of this new feature.

This new feature would also mean that we have to drop edk2's DefconfigPath.

@MDr164 when parsing JSON (or YAML #446), is the order of elements in JSON array preserved when fed into go's array/slice? My commons sense says it should be preserved, I just want to confirm.