make: fix CPU reset in some conditions

make: fix CPU reset in some conditions

In some cases like trying to build a package with debug config
(e.g. make cfg mode=debug; .... ; make deb), the CPU make variable
was reset resulting in a build failure. This happens because in
the debug config CPU is not set and hence is  saved as empty.
Trying to build a package will start a sub-make which will try to
re-do the config without debugging (make cfg), but the CPU
variable with and empty value will be inherited from the
environment and thus the CPU?=something lines won't have any
effect.
The fix replaces CPU?=something with
$(call set_if_empty,CPU,something)
This works around having an empty CPU in the environment.

Reported-by: Jan Janak  <jan@iptel.org>