cmd: pxe_utils: sysboot: fix crash if either board or soc are not set.

If the environment does not have "soc" or "board" set, and fdtdir
option is specified in extlinux.conf, the bootloader will crash whilst
dereferencing a null pointer. Add a guard against null soc or
board. Fixes a crash of qemu-riscv64_smode configuration, which does
not have CONFIG_SYS_SOC defined.

Signed-off-by: Dimitri John Ledkov <xnox@ubuntu.com>
Reviewed-by: Ramon Fried <rfried.dev@gmail.com>