Move `default-if-blank` and `error-if-blank` to the new `var` namespace (eg `bmakelib.var.default-if-blank`) Filenames and doc links should also be updated as it makes sense to either move them to a single `var.mk` file or prefix each existing file w/ `var.`.
Move
default-if-blankanderror-if-blankto the newvarnamespace (egbmakelib.var.default-if-blank)Filenames and doc links should also be updated as it makes sense to either move them to a single
var.mkfile or prefix each existing file w/var..