libpayload/x86: Fix boot_device_read() and hook it up

Casts from integer to pointer are usually a case for phys_to_virt().

Change-Id: I861d435ff2361cdc26a2abd46d43b9346fa67ccc
Signed-off-by: Nico Huber <nico.h@gmx.de>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/62246
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Julius Werner <jwerner@chromium.org>

payloads/libpayload/arch/x86/Makefile.inc

@@ -42,6 +42,7 @@ libc-$(CONFIG_LP_GPL) += string.c
 libgdb-y += gdb.c
 
 libcbfs-$(CONFIG_LP_CBFS) += rom_media.c
+libcbfs-$(CONFIG_LP_CBFS) += boot_media.c
 
 # Multiboot support is configurable
 libc-$(CONFIG_LP_MULTIBOOT) += multiboot.c