Librem 5: u-boot-librem5 depends on newlib-arm-none-eabi from Alpine testing

Currently we can't put the Librem 5 in the stable branch of postmarketOS, which builds against a stable release of Alpine. The stable releases of Alpine don't have the testing subdir of their aports.

It's also not trivial to fork newlib-arm-none-eabi, as this in turn depends on gcc-arm-none-eabi from Alpine testing, which is part of gcc-cross-embedded and depends on gcc-arm-none-eabi itself.

In order to not block the v20.05 beta release, I'll delete u-boot-librem5 from that branch for now.

Can't we simply use gcc-armv7 from postmarketOS together with the pmb:cross-native APKBUILD option to build u-boot-librem5 and avoid the dependency altogether?

Related: #551

CC: @craftyguy