build: fix extreme build system slowdown caused by SOURCE_DATE_EPOCH changes

Adding inline shell invocations in per-target variables causes them to be
executed over and over again, which causes a significant slowdown.
Fix this by evaluating it only once per package directory

Signed-off-by: Felix Fietkau <nbd@nbd.name>

include/package.mk

@@ -19,6 +19,8 @@ PKG_IREMAP ?= 1
 
 MAKE_J:=$(if $(MAKE_JOBSERVER),$(MAKE_JOBSERVER) $(if $(filter 3.% 4.0 4.1,$(MAKE_VERSION)),-j))
 
+PKG_SOURCE_DATE_EPOCH:=$(if $(DUMP),,$(shell $(TOPDIR)/scripts/get_source_date_epoch.sh $(CURDIR)))
+
 ifeq ($(strip $(PKG_BUILD_PARALLEL)),0)
 PKG_JOBS?=-j1
 else