cmd: pxe_utils: sysboot: add kaslr-seed generation support

this will add kaslrseed keyword to sysboot lable,
when it set, it will request to genarate random number
from hwrng as kaslr-seed.

with this patch exlinux.conf label looks like

label l0
        menu testing
        linux /boot/vmlinuz-5.15.16-arm
        initrd /boot/initramfs-5.15.16-arm.img
        fdtdir /boot/dtbs/5.15.16-arm/
        kaslrseed
        append root=UUID=92ae1e50-eeeb-4c5b-8939-7e1cd6cfb059 ro

Tested on Khadas VIM with kernel 5.16.0-rc5-arm64, Debian 11.

Signed-off-by: Zhang Ning <zhangn1985@qq.com>