initramfs: make compression options not depend on INITRAMFS_SOURCE

Even if INITRAMFS_SOURCE is empty, usr/gen_initramfs.sh generates a
tiny default initramfs, which is embedded in vmlinux.

So, defining INITRAMFS_COMPRESSION* options should be valid irrespective
of INITRAMFS_SOURCE.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

usr/Kconfig

@@ -102,7 +102,6 @@ config RD_LZ4
 
 choice
 	prompt "Built-in initramfs compression mode"
-	depends on INITRAMFS_SOURCE!=""
 	optional
 	help
 	  This option allows you to decide by which algorithm the builtin
@@ -211,7 +210,6 @@ config INITRAMFS_COMPRESSION_LZ4
 endchoice
 
 config INITRAMFS_COMPRESSION
-	depends on INITRAMFS_SOURCE!=""
 	string
 	default ""      if INITRAMFS_COMPRESSION_NONE
 	default ".gz"   if INITRAMFS_COMPRESSION_GZIP