Update boot script for home partition changes

- Remove the setexpr for boot_partition: We now define this in the
default u-boot environment, instead of deriving it from

Change-Id: Idd5d9ba3e13163ae478e7d6e6f6cbc02955c6162
diff --git a/boot.txt b/boot.txt
index 5df0965..cf61a02 100644
--- a/boot.txt
+++ b/boot.txt
@@ -1,7 +1,6 @@
 setenv fdt_addr 0x42000000
 setenv fdt_file mt8167-coral.dtb
-setexpr boot_partition ${rootfs_partition} - 1
 echo Loading image...
 ext2load mmc ${mmcdev}:${boot_partition} ${kerneladdr} boot.img