temp/u-boot-librem5: use pmb:cross-native (MR 1988)

Let pmbootstrap build the package in the native chroot. This is
important for the v21.03 branch, which needs to ship its own
cross-embedded toolchain, as the one from Alpine is only in testing and
therefore not available in the Alpine v3.13 branch that pmOS v21.03 is
based on. By using the native branch, it is enough to build the
cross-embedded toolchain for x86_64, and we can avoid qemu related
errors when attempting to build it for arm architectures.

As a nice benefit, this makes compilation of the package faster on edge,
too.

[ci:skip-build]: already built successfully in CI

temp/u-boot-librem5/APKBUILD

 # Forked from Alpine, so we can build it with Librem 5 phone support
 pkgname=u-boot-librem5
 pkgver=0.8
-pkgrel=0
+pkgrel=1
 # 'librem5' branch in atf repo
 _atfversion="92c2de12d36b31938ce940d5cac3c30a98665237"
 # 'librem5' branch in uboot repo
@@ -13,7 +13,7 @@ url="http://www.denx.de/wiki/U-Boot/"
 arch="aarch64"
 license="GPL-2.0-or-later OFL-1.1 BSD-2-Clause BSD-3-Clause eCos-2.0 IBM-pibs"
 makedepends="$depends_dev bc dtc python3-dev swig binutils coreutils zlib-dev newlib-arm-none-eabi bison flex"
-options="!check !tracedeps"
+options="!check !tracedeps pmb:cross-native"
 source="
 	uboot-imx-${_ubootversion}.tar.gz::https://source.puri.sm/Librem5/uboot-imx/-/archive/${_ubootversion}/uboot-imx-${_ubootversion}.tar.gz
 	m4-${_m4version}.tar.gz::https://source.puri.sm/Librem5/Cortex_M4/-/archive/${_m4version}/Cortex_M4-${_m4version}.tar.gz