imx6: apalis: move set_blkcnt variable into recovery script

I've added `set_blkcnt` environment variable which is needed for every
`mmc write` command as we need to always specify size in block count.

That `set_blkcnt` variable was previously present in the U-Boot's
default environment, but that needed patching U-Boot every time, so that
patch was dropped with 2020.01 version bump.

Signed-off-by: Petr Štetiar <ynezz@true.cz>

target/linux/imx6/image/recovery-apalis

@@ -4,6 +4,7 @@ mmc dev 0 1
 mmc write 0x12100000 0x2 0x800
 
 # flash openwrt-imx6-apalis-squashfs.combined.bin
+setenv set_blkcnt 'setexpr blkcnt ${filesize} + 0x1ff && setexpr blkcnt ${blkcnt} / 0x200'
 run set_blkcnt
 mmc dev 0 0
 mmc write 0x12500000 0 ${blkcnt}