uboot: Define timeout variable

It is possible to set a timeout for automatic bootloading via a
variable.

features.in/uboot/config.mk

@@ -17,7 +17,9 @@ use/uboot: use/kernel/initrd-setup $(UBOOT_TTY)
 ifeq (,$(filter-out riscv64,$(ARCH)))
 	@$(call add,UBOOT_BOOTARGS,earlyprintk debug no_alt_virt_keyboard)
 endif
+	@$(call try,UBOOT_TIMEOUT,50)
 	@$(call xport,UBOOT_BOOTARGS)
+	@$(call xport,UBOOT_TIMEOUT)
 else
 use/uboot: ; @:
 endif

features.in/uboot/image-scripts.d/00-uboot-prepare

@@ -11,7 +11,7 @@ cat > /boot/extlinux/extlinux.conf << EOF
 menu title ALTLinux Boot Options
 
 default linux
-timeout 70
+timeout $GLOBAL_UBOOT_TIMEOUT
 
 label linux
 	kernel /boot/vmlinuz