bootstd: Init the size before reading the devicetree

The implementation in distro_efi_try_bootflow_files() does not pass a
valid size to bootmeth_common_read_file(), so this can fail if the
uninted value happens to be too small.

Fix this.

This was reported by someone but I cannot now find the email.

Signed-off-by: Simon Glass <sjg@chromium.org>