package/pkg-generic.mk: simplify step_pkg_size

Use the same trick in step_pkg_size as the one used in check_bin_arch:
factorize the two $(filter ...) calls into one, checking in one step
the step and whether it's the beginning or end of the step.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>