qfw: store loaded file size in environment variable

At the moment the QEMU firmware command just prints the size of the
loaded binaries on the console.
To go with all the other load methods, and many boot scripts'
expectations, also store the size of the file loaded last in the
environment variable "filesize".
We first put the kernel size in there, but overwrite this with the
initrd size, should we have one, because this is probably the more
prominent user of $filesize (in the booti or bootz command).

Signed-off-by: Andre Przywara <andre.przywara@arm.com>