Convert CONFIG_SYS_MEMTEST_START et al to Kconfig

This converts the following to Kconfig:
   CONFIG_SYS_MEMTEST_START
   CONFIG_SYS_MEMTEST_END

This is removing unused defines and correcting the default value to be
0x0 as we are a hex symbol.

Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/cmd/Kconfig b/cmd/Kconfig
index fd8f022..7747ab5 100644
--- a/cmd/Kconfig
+++ b/cmd/Kconfig
@@ -741,7 +741,7 @@
 
 config SYS_MEMTEST_START
 	hex "default start address for mtest"
-	default 0
+	default 0x0
 	help
 	  This is the default start address for mtest for simple read/write
 	  test. If no arguments are given to mtest, default address is used