pkg.mk: do not use user identity when applying patches

Use fixed identity when applying patches, it fixes issues when they are
not set in the build computer or if HOME is not exported.

The commits are only used in the build system so adding the building user
information is useless.

pkg/pkg.mk

@@ -19,12 +19,13 @@ else
 git-download: $(PKG_BUILDDIR)/.git-downloaded
 endif
 
+GITFLAGS ?= -c user.email=buildsystem@riot -c user.name="RIOT buildsystem"
 GITAMFLAGS ?= --no-gpg-sign --ignore-whitespace
 
 ifneq (,$(wildcard $(PKG_DIR)/patches))
 $(PKG_BUILDDIR)/.git-patched: $(PKG_BUILDDIR)/.git-downloaded $(PKG_DIR)/Makefile $(PKG_DIR)/patches/*.patch
 	git -C $(PKG_BUILDDIR) checkout -f $(PKG_VERSION)
-	git -C $(PKG_BUILDDIR) am $(GITAMFLAGS) "$(PKG_DIR)"/patches/*.patch
+	git $(GITFLAGS) -C $(PKG_BUILDDIR) am $(GITAMFLAGS) "$(PKG_DIR)"/patches/*.patch
 	touch $@
 endif