Convert CONFIG_BOOTFILE to Kconfig

This converts the following to Kconfig:
   CONFIG_BOOTFILE

Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/configs/nsim_hs38be_defconfig b/configs/nsim_hs38be_defconfig
index 60e6094..bd1cdf3 100644
--- a/configs/nsim_hs38be_defconfig
+++ b/configs/nsim_hs38be_defconfig
@@ -19,6 +19,8 @@
 CONFIG_OF_CONTROL=y
 CONFIG_OF_EMBED=y
 CONFIG_SYS_RELOC_GD_ENV_ADDR=y
+CONFIG_USE_BOOTFILE=y
+CONFIG_BOOTFILE="uImage"
 # CONFIG_NET is not set
 CONFIG_DM_SERIAL=y
 CONFIG_DEBUG_UART_SHIFT=2