package/pkg-golang.mk: make GOPROXY configurable

BR2_GOPROXY now configures the Go module proxy URL used to download Go modules
specified in project go.mod and go.sum files. By default, it uses Google's
official Go module mirror at proxy.golang.org, falling back to direct downloads
if the module is not found in the mirror.

The default value from the toolchain can be read with "go env GOPROXY"

We rely on package developers to audit their dependencies in go.mod and go.sum.

The value can be configured to:
- A comma-separated list of proxy URLs
- "off" to disable all downloads
- "direct" to download directly from source repositories

For more details on Go module proxies:
https://go.dev/ref/mod#module-proxy

Discussion:
https://lore.kernel.org/all/CADvTj4pXO2rwJ8oCzzGNY+OxLuo3uJnDT2PAxbyquw0L6dhfiA@mail.gmail.com/T/#t

Signed-off-by: James Hilliard <james.hilliard1@gmail.com>
Signed-off-by: Christian Stewart <christian@aperture.us>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

Config.in

@@ -334,6 +334,22 @@ config BR2_LUAROCKS_MIRROR
 
 	  See http://luarocks.org
 
+config BR2_GOPROXY
+	string "Go module proxy"
+	default "https://proxy.golang.org,direct"
+	depends on BR2_PACKAGE_HOST_GO_TARGET_ARCH_SUPPORTS
+	help
+	  The Go module proxy URL used to download Go modules. By
+	  default, it uses the official Go module mirror, falling back
+	  to direct downloads if the module is not found in the mirror.
+
+	  The value can be one of:
+	    - Comma-separated list of proxy URLs
+	    - "off" to disable all downloads
+	    - "direct" to download directly from source repositories
+
+	  See https://go.dev/ref/mod#module-proxy
+
 config BR2_CPAN_MIRROR
 	string "CPAN mirror (Perl packages)"
 	default "https://cpan.metacpan.org"

package/pkg-golang.mk

@@ -101,7 +101,7 @@ $(2)_POST_PATCH_HOOKS += $(2)_GEN_GOMOD
 $(2)_DOWNLOAD_POST_PROCESS = go
 $(2)_DL_ENV += \
 	$$(HOST_GO_COMMON_ENV) \
-	GOPROXY="https://proxy.golang.org,direct" \
+	GOPROXY=$$(call qstrip,$$(BR2_GOPROXY)) \
 	$$($(2)_GO_ENV)
 
 # If building in a sub directory, do the vendoring in there