treewide: Migrate CONFIG_SYS_ALT_MEMTEST to Kconfig

Migrate the CONFIG_SYS_ALT_MEMTEST option to Kconfig.

Signed-off-by: Mario Six <mario.six@gdsys.cc>
[trini: Re-run migration after also including CMD_MEMTEST]
Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/configs/ap121_defconfig b/configs/ap121_defconfig
index d88be90..2d418f6 100644
--- a/configs/ap121_defconfig
+++ b/configs/ap121_defconfig
@@ -18,6 +18,7 @@
 # CONFIG_CMD_IMPORTENV is not set
 # CONFIG_CMD_EDITENV is not set
 # CONFIG_CMD_CRC32 is not set
+CONFIG_CMD_MEMTEST=y
 # CONFIG_CMD_FLASH is not set
 CONFIG_CMD_SF=y
 CONFIG_CMD_SPI=y