env: ti: boot: Fix Android boot on AM57x EVM

env: ti: boot: Fix Android boot on AM57x EVM

When applying DTBO on top of DTB (with "fdt apply" command) on AM57x EVM
board, there is not enough memory reserved in RAM for DTB blob. Hence,
DTBO can't be merged in DTB. It leads to inability to boot Android with
next error message:

    failed on fdt_overlay_apply(): FDT_ERR_NOSPACE

To overcome that issue let's provide 512 KiB of space to keep DTB and
all merged DTBO blobs. To do so, "length" parameter should be specified
for "fdt addr" command:

    => fdt addr $fdtaddr 0x80000

512 KiB is the maximum size we can use for this, because next address
after $fdtaddr is 512 KiB ahead of it:

    fdtaddr=0x88000000
    rdaddr=0x88080000

Also add size variables to 'adtimg' command invocations, to avoid
cluttering the console with DTBO blob sizes.

Signed-off-by: Sam Protsenko <joe.skb7@gmail.com>
Reviewed-by: Eugeniu Rosca <rosca.eugeniu@gmail.com>