x86: add bootloader upgrade on sysupgrade

Currently bootloader always stays on the same version as when first
written to boot medium (not true if partition layout changed, which will
trigger sysupgrade process to write full disk image). That creates
inconveniences as it always stays with same features or/and bugs. Users
wishing to add support to additional modules or new version, would need
to write the whole image, potentially destroying previous system
configuration. To fix these, this commit adds additional routine to
sysupgrade which upgrades unconditionally the bootloader to the latest
state provided by OpenWrt.

Signed-off-by: Tomasz Maciej Nowak <tomek_n@o2.pl>