[source] image.mk: add generic function for gzipping images if enabled

[LEDE Commits]

nbd pushed a commit to source.git, branch master:
https://git.lede-project.org/8af5e5751d230fda28a0921f1e312b5090a5ac97

commit 8af5e5751d230fda28a0921f1e312b5090a5ac97
Author: Felix Fietkau <nbd at nbd.name>
AuthorDate: Fri Jan 13 16:19:00 2017 +0100

    image.mk: add generic function for gzipping images if enabled
    
    Signed-off-by: Felix Fietkau <nbd at nbd.name>
---
 include/image.mk | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/include/image.mk b/include/image.mk
index 4bd923f..81519cd 100644
--- a/include/image.mk
+++ b/include/image.mk
@@ -147,6 +147,13 @@ define Image/BuildKernel/MkFIT
 	PATH=$(LINUX_DIR)/scripts/dtc:$(PATH) mkimage -f $(KDIR)/fit-$(1).its $(KDIR)/fit-$(1)$(7).itb
 endef
 
+ifdef CONFIG_TARGET_IMAGES_GZIP
+  define Image/Gzip
+	rm -f $(1).gz
+	gzip -9n $(1)
+  endef
+endif
+
 # $(1) source dts file
 # $(2) target dtb file
 # $(3) extra CPP flags