GitHub Action: add release script (#124)

This script will be triggered by any tag starting with `v[0-9]+.[0-9]+.[0-9]+` (i.e.
v1.1.0). It will create all of the .tar.gz files (or .zip for windows). The amd64 binaries will be
compiled on their target systems, the rest of the Linux architecures
will be cross compiled from the Linux amd64 host.

A SHASUM256.txt will also be generated and attached to the release.

Makefile

@@ -3,7 +3,7 @@ BUILD_NUMBER ?= dev+$(shell date -u '+%Y%m%d%H%M%S')
 GO111MODULE = on
 export GO111MODULE
 
-ALL = linux-amd64 \
+ALL_LINUX = linux-amd64 \
 	linux-386 \
 	linux-ppc64le \
 	linux-arm-5 \
@@ -13,7 +13,9 @@ ALL = linux-amd64 \
 	linux-mips \
 	linux-mipsle \
 	linux-mips64 \
-	linux-mips64le \
+	linux-mips64le
+
+ALL = $(ALL_LINUX) \
 	darwin-amd64 \
 	windows-amd64
 
@@ -21,6 +23,8 @@ all: $(ALL:%=build/%/nebula) $(ALL:%=build/%/nebula-cert)
 
 release: $(ALL:%=build/nebula-%.tar.gz)
 
+release-linux: $(ALL_LINUX:%=build/nebula-%.tar.gz)
+
 bin:
 	go build -trimpath -ldflags "-X main.Build=$(BUILD_NUMBER)" -o ./nebula ${NEBULA_CMD_PATH}
 	go build -trimpath -ldflags "-X main.Build=$(BUILD_NUMBER)" -o ./nebula-cert ./cmd/nebula-cert