bootretry: check for bootretry variable changes

bootretry: check for bootretry variable changes

To enable more complex sequencing of the bootmenu, autoboot, and
bootretry, handle changes to the bootretry variable between tries. This
makes it possible to turn bootretry off (e.g. to drop to a shell) and
then back on again.

This makes it possible to have a persistent bootmenu (the only way to
navigate U-Boot on devices like smartphones which lack a physical
keyboard) by having bootcmd be defined to launch the bootmenu. This
allows for menu options like enabling USB mass storage gadget to return
back to the boot menu once the gadget is shut down.

Reviewed-by: Tom Rini <trini@konsulko.com>
Tested-by: Danila Tikhonov <danila@jiaxyga.com> # google-sunfish
Tested-by: Jens Reidel <adrian@mainlining.org> # xiaomi-davinci
Link: https://lore.kernel.org/r/20250331-qcom-phones-v4-3-f52e57d3b8c6@linaro.org
Signed-off-by: Caleb Connolly <caleb.connolly@linaro.org>