squashfs: use kconfig to ensure atleast one compressor is selected

Similar to how we do it for mpd now. This way, the selection is clearly
visible to the user, instead of the hidden selection we used to do in
the makefile.

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>

package/squashfs/Config.in

@@ -1,6 +1,7 @@
 config BR2_PACKAGE_SQUASHFS
 	bool "squashfs"
 	depends on BR2_LARGEFILE
+	select BR2_PACKAGE_SQUASHFS_GZIP if !(BR2_PACKAGE_SQUASHFS_LZMA || BR2_PACKAGE_SQUASHFS_LZO)
 	help
 	  Tools to generate SquashFS filesystems.
 

package/squashfs/squashfs.mk

@@ -5,11 +5,6 @@ SQUASHFS_SITE=http://$(BR2_SOURCEFORGE_MIRROR).dl.sourceforge.net/sourceforge/sq
 # no libattr/xz in BR
 SQUASHFS_MAKE_ARGS = XATTR_SUPPORT=0
 
-# we need atleast one compresser, so use gzip if none are enabled
-ifeq ($(BR2_PACKAGE_SQUASHFS_GZIP)$(BR2_PACKAGE_SQUASHFS_LZO)$(BR2_PACKAGE_SQUASHFS_LZMA),)
-BR2_PACKAGE_SQUASHFS_GZIP=y
-endif
-
 ifeq ($(BR2_PACKAGE_SQUASHFS_LZMA),y)
 SQUASHFS_DEPENDENCIES += xz
 SQUASHFS_MAKE_ARGS += XZ_SUPPORT=1 COMP_DEFAULT=lzma