bootmeth_extlinux: Refactor extlinux to split boot

Add a new extlinux_setup() function which sets things up so that the
bootflow can either be booted, or just probed.

Provide a readall() method so that images can be read into memory,
without booting the OS. Adjust the boot() method so that it can boot,
after any changes have been made by the user.

Update the test to change the bootargs and check that they are booted
with the changes intact.

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