Improve the Makefile speed by simple assignment

In slow network environments, recursive variable assignments (`=`)
in the Makefile can cause shell commands, such as `wget` for
`LINUX_DATA`, to execute multiple times,
significantly slowing down the build process.

By using simple assignment (`:=`), `wget` is executed only once
during Makefile parsing. This reduces redundant network requests
and improves overall Makefile execution speed.

Author: eastWillow

mk/external.mk

@@ -128,12 +128,12 @@ LINUX_CDN_BASE_URL = https://cdn.kernel.org/pub/linux/kernel
 LINUX_CDN_VERSION_URL = $(LINUX_CDN_BASE_URL)/v$(LINUX_VERSION).x
 $(shell mkdir -p /tmp/linux)
 LINUX_DATA_DEST = /tmp/linux
-LINUX_DATA = $(shell wget -q -O- $(LINUX_CDN_VERSION_URL) | \
+LINUX_DATA := $(shell wget -q -O- $(LINUX_CDN_VERSION_URL) | \
                      grep -o 'linux-$(LINUX_VERSION).$(LINUX_PATCHLEVEL).[0-9]\+\.tar.gz' | \
                      sort -V | tail -n 1)
 LINUX_DATA_URL = $(LINUX_CDN_VERSION_URL)/$(LINUX_DATA)
 LINUX_DATA_SKIP_DIR_LEVEL = 1
-LINUX_DATA_SHA = $(shell wget -q -O- $(LINUX_CDN_VERSION_URL)/sha256sums.asc | \
+LINUX_DATA_SHA := $(shell wget -q -O- $(LINUX_CDN_VERSION_URL)/sha256sums.asc | \
                          grep $(LINUX_DATA) | awk '{print $$1}')
 LINUX_DATA_SHA_CMD = $(SHA256SUM)