boot: Update SYS_BOOTM_LEN to depend on BOOTM

Use the new CONFIG_BOOTM symbol to determine whether SYS_BOOT_LEN is
visible or not, since we want to support decompression when CMDLINE is
disabled.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/boot/Kconfig b/boot/Kconfig
index 9fa8789..d83047a 100644
--- a/boot/Kconfig
+++ b/boot/Kconfig
@@ -800,7 +800,7 @@
 
 config SYS_BOOTM_LEN
 	hex "Maximum size of a decompresed OS image"
-	depends on CMD_BOOTM || CMD_BOOTI || CMD_BOOTZ || \
+	depends on BOOTM || CMD_BOOTI || CMD_BOOTZ || \
 		LEGACY_IMAGE_FORMAT || SPL_LEGACY_IMAGE_FORMAT
 	default 0x4000000 if PPC || ARM64
 	default 0x1000000 if X86 || ARCH_MX6 || ARCH_MX7