Makefile: add EXTRA_LDFLAGS

[sandy-lcq]

Add EXTRA_LDFLAGS to support add extra ldflags

[kolyshkin]

@sandy-lcq why can't you use existing EXTRA_FLAGS? E.g.

make EXTRA_FLAGS="..."

[sandy-lcq]

Hi,

with the old BUILD_FLAGS := -tags "$(BUILDTAGS)" -ldflags "-X main.gitCommit=$(COMMIT) -X main.version=$(VERSION)" $(EXTRA_FLAGS)

if user want to pass some config for -ldflags, user can only pass like EXTRA_FLAGS="-ldflags -buildid=", in this way, the latter option will overrides the first "-ldflag "-X main.gitCommit=$(COMMIT) -X main.version=$(VERSION)"", unless I pass like
EXTRA_FLAGS ="-ldflags -X main.gitCommit=$(COMMIT) -X main.version=$(VERSION)" -buildid=", so I think maybe add another EXTRA_LDFLAGS is better. So user can only set EXTRA_LDFLAGS="-buildid="