Convert CONFIG_SYS_PROMPT et al to Kconfig

This converts the following to Kconfig:
   CONFIG_SYS_PROMPT
   CONFIG_SYS_PROMPT_HUSH_PS2

Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/configs/imx8mq_evk_defconfig b/configs/imx8mq_evk_defconfig
index b02d9c1..cf6068c 100644
--- a/configs/imx8mq_evk_defconfig
+++ b/configs/imx8mq_evk_defconfig
@@ -30,6 +30,7 @@
 CONFIG_SPL_I2C=y
 CONFIG_SPL_POWER=y
 CONFIG_SPL_WATCHDOG=y
+CONFIG_SYS_PROMPT="u-boot=> "
 # CONFIG_BOOTM_NETBSD is not set
 # CONFIG_CMD_EXPORTENV is not set
 # CONFIG_CMD_IMPORTENV is not set