ipxe: Export SOURCE_DATE_EPOCH so builds are reproducible

I was seeing reproducibility from 2 sources, recording the build
timestamp and injecting random numbers for BUILD_ID. I've fixed
both upstream in ipxe/ipxe#234

Signed-off-by: Manuel Mendez <mmendez@equinix.com>

rules.mk

@@ -17,6 +17,9 @@ CGO_ENABLED := 0
 export CGO_ENABLED
 
 GitRev := $(shell git rev-parse --short HEAD)
+SOURCE_DATE_EPOCH := $(shell git log -1 --pretty=%ct)
+export SOURCE_DATE_EPOCH
+
 crossbinaries := cmd/boots/boots-linux-386 cmd/boots/boots-linux-amd64 cmd/boots/boots-linux-arm64 cmd/boots/boots-linux-armv6 cmd/boots/boots-linux-armv7
 cmd/boots/boots-linux-386:   FLAGS=GOARCH=386
 cmd/boots/boots-linux-amd64: FLAGS=GOARCH=amd64