temp/u-boot-librem5: options=pmb:cross-native

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.

@craftyguy, @samuelnorbury2: can you verify that the u-boot build from this works as expected?

Related: #598 (closed)