Convert CONFIG_ENV_IS_IN_SPI_FLASH to Kconfig

This converts the following to Kconfig:
   CONFIG_ENV_IS_IN_SPI_FLASH

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/configs/trimslice_defconfig b/configs/trimslice_defconfig
index cb42ac5..a904e4b 100644
--- a/configs/trimslice_defconfig
+++ b/configs/trimslice_defconfig
@@ -4,6 +4,7 @@
 CONFIG_TARGET_TRIMSLICE=y
 CONFIG_DEFAULT_DEVICE_TREE="tegra20-trimslice"
 CONFIG_OF_SYSTEM_SETUP=y
+CONFIG_ENV_IS_IN_SPI_FLASH=y
 CONFIG_CONSOLE_MUX=y
 CONFIG_SYS_STDIO_DEREGISTER=y
 CONFIG_SYS_PROMPT="Tegra20 (TrimSlice) # "