bootstd: Support reading a script from network or SPI flash

At present this bootmeth only supports a block device and the sandbox
host filesystem. Add support for obtaining the script from a network
device. Also implement the set_bootflow() method so that it is easy
for other bootdevs (such as enabling SPI flash to support scripts).

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