[BSP][UBOOT] add config to disable bootdelay

Yconfig: add choice block with four options:

• disable bootdelay
• disable autoboot entirely
• enable bootdelay=0 (boot immediately but allow interrupting)
• enable bootdelay=3 (wait 3 seconds, then boot)

This is done with four different configuration fragments because the file-parametric bbclass does not yet support configuration parameterizing configuration fragments.

The "disable bootdelay" option is useful for Raspberry Pi CM5 that, because of this issue: https://lists.denx.de/pipermail/u-boot/2024-December/576043.html receives spurious input from the UART and interrupts autoboot.

This commit also introduces a new configuration menu: "Generic BSP configuration"

Signed-off-by: Alessandro Pecugi alessandro.pecugi@seco.com