icecc: Don't replace non-empty PARALLEL_MAKE with empty ICECC_PARALLEL_MAKE
* it's needed for use-case like this:
# Inherit icecc here, so that all builders have the same sstate signatures
INHERIT_DISTRO += "icecc"
# and then disable its function by default (so that people still need to explicity
# enable it in local.conf if they have configured icecc and want to use it.
# You need to set _empty_ value in local.conf to enable icecc function:
# ICECC_DISABLED = ""
ICECC_DISABLED ??= "1"
* so default ICECC_PARALLEL_MAKE is still empty, but we want build
to respect our PARALLEL_MAKE, unfortunately we cannot do something
like ICECC_PARALLEL_MAKE ??= "${PARALLEL_MAKE}", because that would
cause PARALLEL_MAKE to reference itself.
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com> Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
