Increase block size of kernel squashfs image to 128K (#3509)

While mksquashfs uses this value by default, Genimage's default is 4K. This is
far too low value and results in slower kernel load, especially on embedded
boards with a flash drive. Explicitly set it to 128K to generate same images as
in pre-genimage builds.

(cherry picked from commit edba18f)

buildroot-external/genimage/images-os.cfg

@@ -4,6 +4,7 @@ image kernel.img {
 
 	squashfs {
 		compression = "lzo"
+		block-size = 131072
 	}
 }