go.bbclass: Enable parallel build

The parallel build is based on PARALLEL_MAKE variable but can be
overriden setting the GO_PARALLEL_BUILD one.

This change is based on the meta-golang[1] layer. Thanks to Matt
Madison <matt@madison.systems> for his work on this.

1. https://github.com/madisongh/meta-golang

(From OE-Core rev: 7a40ea8fc358aa134ad86862591329f94d0f3718)

Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/classes/go.bbclass

@@ -1,11 +1,31 @@
 inherit goarch
 
+def get_go_parallel_make(d):
+    pm = (d.getVar('PARALLEL_MAKE') or '').split()
+    # look for '-j' and throw other options (e.g. '-l') away
+    # because they might have a different meaning in golang
+    while pm:
+        opt = pm.pop(0)
+        if opt == '-j':
+            v = pm.pop(0)
+        elif opt.startswith('-j'):
+            v = opt[2:].strip()
+        else:
+            continue
+
+        return '-p %d' % int(v)
+
+    return ""
+
+GO_PARALLEL_BUILD ?= "${@get_go_parallel_make(d)}"
+
 GOROOT_class-native = "${STAGING_LIBDIR_NATIVE}/go"
 GOROOT = "${STAGING_LIBDIR_NATIVE}/${TARGET_SYS}/go"
 GOBIN_FINAL_class-native = "${GOROOT_FINAL}/bin"
 GOBIN_FINAL = "${GOROOT_FINAL}/${GO_BUILD_BINDIR}"
 
 export GOBUILDFLAGS ?= "-v"
+GOBUILDFLAGS_prepend_task-compile = "${GO_PARALLEL_BUILD} "
 
 export GOOS = "${TARGET_GOOS}"
 export GOARCH = "${TARGET_GOARCH}"