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/nitrogen6s1g_defconfig b/configs/nitrogen6s1g_defconfig
index e7348af..7000c52 100644
--- a/configs/nitrogen6s1g_defconfig
+++ b/configs/nitrogen6s1g_defconfig
@@ -13,6 +13,7 @@
 CONFIG_FASTBOOT_BUF_ADDR=0x12000000
 CONFIG_CMD_BOOTZ=y
 CONFIG_CMD_MEMTEST=y
+CONFIG_SYS_ALT_MEMTEST=y
 # CONFIG_CMD_FLASH is not set
 CONFIG_CMD_GPIO=y
 CONFIG_CMD_I2C=y