Fix binary names (#122)

## Description

This PR fixes the binary names to be both correct and also "unsurprising".

## Why is this needed

Avoids needing to COPY all the binaries, only to keep 1. Also avoids glaringly misnamed binaries :D. 

For example these are the binaries built from master:
```
file boots-* | sed 's|, statically.*||'
boots-linux-aarch64: ELF 32-bit LSB executable, ARM, EABI5 version 1 (SYSV)
boots-linux-amd64:   ELF 64-bit LSB executable, x86-64, version 1 (SYSV)
boots-linux-arm64:   ELF 64-bit LSB executable, ARM aarch64, version 1 (SYSV)
boots-linux-armv7l:  ELF 32-bit LSB executable, ARM, EABI5 version 1 (SYSV)
boots-linux-x86_64:  ELF 32-bit LSB executable, Intel 80386, version 1 (SYSV)
```
notice the 32bit arm v6 binary is named aarch64 and x86_64 is actually 386!

Here is from this PR:
```
[~/go/src/github.com/tinkerbell/boots]─[manny@dellnix]> 
file boots-* | sed 's|, statically.*||'
boots-linux-386:   ELF 32-bit LSB executable, Intel 80386, version 1 (SYSV)
boots-linux-amd64: ELF 64-bit LSB executable, x86-64, version 1 (SYSV)
boots-linux-arm64: ELF 64-bit LSB executable, ARM aarch64, version 1 (SYSV)
boots-linux-armv6: ELF 32-bit LSB executable, ARM, EABI5 version 1 (SYSV)
boots-linux-armv7: ELF 32-bit LSB executable, ARM, EABI5 version 1 (SYSV)
```

## How Has This Been Tested?

Inspecting each file with `file`, docker build works.

## How are existing users impacted? What migration steps/scripts do we need?

Any user using the cross compiled binaries directly will be impacted, but this should most likely be 0 users since they will likely use plain-old `boots` binary instead.

.dockerignore

@@ -0,0 +1,2 @@
+*
+!boots-*-*

.gitignore

@@ -4,3 +4,4 @@
 .idea*/**
 /bin/
 boots
+boots-*-*

Dockerfile

@@ -1,7 +1,11 @@
 FROM alpine:3.11
+
+ARG TARGETOS
+ARG TARGETARCH
+ARG TARGETVARIANT
+
 ENTRYPOINT ["/usr/bin/boots"]
 EXPOSE 67 69 80
+
 RUN apk add --update --upgrade --no-cache ca-certificates socat
-COPY . /usr/myapp
-RUN cp /usr/myapp/boots-linux-$(uname -m) /usr/bin/boots
-RUN rm -fr /usr/myapp
+COPY boots-${TARGETOS:-linux}-${TARGETARCH:-amd64}${TARGETVARIANT} /usr/bin/boots

Makefile

@@ -15,18 +15,23 @@ binary := boots
 .PHONY: all ${binary} crosscompile dc gen run test
 all: ${binary}
 
+CGO_ENABLED := 0
+export CGO_ENABLED
+
 GitRev := $(shell git rev-parse --short HEAD)
-crosscompile: ${binary}
-	CGO_ENABLED=0 GOOS=linux GOARCH=386 go build -v -o ./boots-linux-x86_64 -ldflags="-X main.GitRev=${GitRev}"
-	CGO_ENABLED=0 GOOS=linux GOARCH=amd64 go build -v -o ./boots-linux-amd64 -ldflags="-X main.GitRev=${GitRev}"
-	CGO_ENABLED=0 GOOS=linux GOARCH=arm GOARM=6 go build -v -o ./boots-linux-aarch64 -ldflags="-X main.GitRev=${GitRev}"
-	CGO_ENABLED=0 GOOS=linux GOARCH=arm GOARM=7 go build -v -o ./boots-linux-armv7l -ldflags="-X main.GitRev=${GitRev}"
-	CGO_ENABLED=0 GOOS=linux GOARCH=arm64 go build -v -o ./boots-linux-arm64 -ldflags="-X main.GitRev=${GitRev}"
+crosscompile: boots-linux-386 boots-linux-amd64 boots-linux-arm64 boots-linux-armv6 boots-linux-armv7
+boots-linux-386:   FLAGS=GOARCH=386
+boots-linux-amd64: FLAGS=GOARCH=amd64
+boots-linux-arm64: FLAGS=GOARCH=arm64
+boots-linux-armv6: FLAGS=GOARCH=arm GOARM=6
+boots-linux-armv7: FLAGS=GOARCH=arm GOARM=7
+boots-linux-386 boots-linux-amd64 boots-linux-arm64 boots-linux-armv6 boots-linux-armv7: ${binary}
+	${FLAGS} GOOS=linux go build -v -ldflags="-X main.GitRev=${GitRev}" -o $@
 
 # this is quick and its really only for rebuilding when dev'ing, I wish go would
 # output deps in make syntax like gcc does... oh well this is good enough
 ${binary}: $(shell git ls-files | grep -v -e vendor -e '_test.go' | grep '.go$$' ) ipxe/bindata.go
-	CGO_ENABLED=0 go build -v -ldflags="-X main.GitRev=${GitRev}"
+	go build -v -ldflags="-X main.GitRev=${GitRev}"
 
 ifeq ($(origin GOBIN), undefined)
 GOBIN := ${PWD}/bin