Convert CONFIG_SYS_BOOTPARAMS_LEN to Kconfig

This converts the following to Kconfig:
   CONFIG_SYS_BOOTPARAMS_LEN

Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/configs/mt7620_rfb_defconfig b/configs/mt7620_rfb_defconfig
index ac7a56e..8a4124e 100644
--- a/configs/mt7620_rfb_defconfig
+++ b/configs/mt7620_rfb_defconfig
@@ -22,6 +22,7 @@
 CONFIG_DEBUG_UART=y
 CONFIG_FIT=y
 # CONFIG_ARCH_FIXUP_FDT_MEMORY is not set
+CONFIG_SYS_MALLOC_BOOTPARAMS=y
 CONFIG_SPL_SYS_MALLOC_SIMPLE=y
 CONFIG_SPL_NOR_SUPPORT=y
 # CONFIG_CMD_ELF is not set