tools/bpftool: Fix build slowdown

mainline inclusion
from mainline-5.11-rc1
commit 0639e5e9
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I5EUVD
CVE: NA

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=0639e5e97ad9c58dd15dcf6f6ccf677cfba39f98

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Commit ba2fd563 ("tools/bpftool: Support passing BPFTOOL_VERSION to
make") changed BPFTOOL_VERSION to a recursively expanded variable,
forcing it to be recomputed on every expansion of CFLAGS and
dramatically slowing down the bpftool build. Restore BPFTOOL_VERSION as
a simply expanded variable, guarded by an ifeq().

Fixes: ba2fd563 ("tools/bpftool: Support passing BPFTOOL_VERSION to make")
Signed-off-by: NJean-Philippe Brucker <jean-philippe@linaro.org>
Signed-off-by: NAndrii Nakryiko <andrii@kernel.org>
Acked-by: NAndrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20201110164310.2600671-8-jean-philippe@linaro.org
(cherry picked from commit 0639e5e9)
Signed-off-by: NWang Yufen <wangyufen@huawei.com>

tools/bpf/bpftool/Makefile

@@ -29,7 +29,9 @@ LIBBPF = $(LIBBPF_PATH)libbpf.a
 LIBBPF_BOOTSTRAP_OUTPUT = $(BOOTSTRAP_OUTPUT)libbpf/
 LIBBPF_BOOTSTRAP = $(LIBBPF_BOOTSTRAP_OUTPUT)libbpf.a
 
-BPFTOOL_VERSION ?= $(shell make -rR --no-print-directory -sC ../../.. kernelversion)
+ifeq ($(BPFTOOL_VERSION),)
+BPFTOOL_VERSION := $(shell make -rR --no-print-directory -sC ../../.. kernelversion)
+endif
 
 $(LIBBPF_OUTPUT) $(BOOTSTRAP_OUTPUT) $(LIBBPF_BOOTSTRAP_OUTPUT):
 	$(QUIET_MKDIR)mkdir -p $@