main/opensbi: add comment regarding u-boot rebuilds

u-boot now depends on OpenSBI and embeds the OpenSBI fw_dynamic.bin
payload into the generated u-boot bootloader image. For this reason,
u-boot needs to be rebuild when changes to OpenSBI are made.

main/opensbi/APKBUILD

 # Contributor: Milan P. Stanić <mps@arvanta.net>
 # Maintainer: Milan P. Stanić <mps@arvanta.net>
 pkgname=opensbi
+# XXX: Updates require rebuilds of main/u-boot.
 pkgver=0.9
 pkgrel=0
 pkgdesc="risc-v open source supervisor binary interface"