imx: imx6ul: kontron-sl-mx6ul: Migrate to use CONFIG_EXTRA_ENV_TEXT

Move the environment from the board header to a separate text file
and also drop those variables that are already set in env_default.h
from the Kconfig options or are not needed anymore.

Signed-off-by: Frieder Schrempf <frieder.schrempf@kontron.de>
diff --git a/board/kontron/sl-mx6ul/sl-mx6ul.env b/board/kontron/sl-mx6ul/sl-mx6ul.env
new file mode 100644
index 0000000..9484e73
--- /dev/null
+++ b/board/kontron/sl-mx6ul/sl-mx6ul.env
@@ -0,0 +1,4 @@
+kernel_addr_r=0x82000000
+ramdisk_addr_r=0x88080000
+pxefile_addr_r=0x80100000
+scriptaddr=0x80100000