kernel: speed up build system by getting rid of redundant work

KERNELRELEASE contains a $(shell) call which is evaluated over and over
again.
The call to checksyscalls.sh is unnecessary for LEDE and also takes a
few seconds to complete.

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

include/kernel-defaults.mk

@@ -23,7 +23,9 @@ KERNEL_MAKEOPTS := -C $(LINUX_DIR) \
 	HOST_LOADLIBES="-L$(STAGING_DIR_HOST)/lib" \
 	CONFIG_SHELL="$(BASH)" \
 	$(if $(findstring c,$(OPENWRT_VERBOSE)),V=1,V='') \
-	$(if $(PKG_BUILD_ID),LDFLAGS_MODULE=--build-id=0x$(PKG_BUILD_ID))
+	$(if $(PKG_BUILD_ID),LDFLAGS_MODULE=--build-id=0x$(PKG_BUILD_ID)) \
+	KERNELRELEASE=$(LINUX_VERSION) \
+	cmd_syscalls=
 
 ifdef CONFIG_STRIP_KERNEL_EXPORTS
   KERNEL_MAKEOPTS += \