cfg.mk: Simplify backslash alignment check

cfg.mk: Simplify backslash alignment check

The use of [[:blank:]] was intended to cover tabs as well, but
it couldn't possibly work in its current form, so the regex was
tweaked in d09429abe826.

With the original reason for using [[:blank:]] now gone, we can
replace its usage with plain spaces. A comment about the purpose
of the check is added as well.

Signed-off-by: Andrea Bolognani <abologna@redhat.com>