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 <[email protected]>

include/package-ipkg.mk

@@ -181,7 +181,7 @@ $$(call addfield,Depends,$$(Package/$(1)/DEPENDS)
 )$$(call addfield,LicenseFiles,$(LICENSE_FILES)
 )$$(call addfield,Section,$(SECTION)
 )$$(call addfield,Require-User,$(USERID)
-)$$(call addfield,SourceDateEpoch,$(shell $(TOPDIR)/scripts/get_source_date_epoch.sh $(SOURCE))
+)$$(call addfield,SourceDateEpoch,$(PKG_SOURCE_DATE_EPOCH)
 )$(if $(PKG_CPE_ID),CPE-ID: $(PKG_CPE_ID)
 )$(if $(filter hold,$(PKG_FLAGS)),Status: unknown hold not-installed
 )$(if $(filter essential,$(PKG_FLAGS)),Essential: yes
@@ -193,7 +193,7 @@ $(_endef)
     $$(IPKG_$(1)) : export CONTROL=$$(Package/$(1)/CONTROL)
     $$(IPKG_$(1)) : export DESCRIPTION=$$(Package/$(1)/description)
     $$(IPKG_$(1)) : export PATH=$$(TARGET_PATH_PKG)
-    $$(IPKG_$(1)) : export PKG_SOURCE_DATE_EPOCH=$(shell $(TOPDIR)/scripts/get_source_date_epoch.sh $(SOURCE))
+    $$(IPKG_$(1)) : export PKG_SOURCE_DATE_EPOCH:=$(PKG_SOURCE_DATE_EPOCH)
     $(PKG_INFO_DIR)/$(1).provides $$(IPKG_$(1)): $(STAMP_BUILT) $(INCLUDE_DIR)/package-ipkg.mk
 	@rm -rf $$(IDIR_$(1)); \
 		$$(call remove_ipkg_files,$(1),$$(call opkg_package_files,$(call gen_ipkg_wildcard,$(1))))

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