sd-bootscript: Set PACKAGE_ARCH to MACHINE

Bootscripts are machine specific, make this explicit.
maxam: Add "sf probe" command to SD bootscript

Apparently probing the flash has side effects. Without it, the flash
fails to work when booting from SD card. Work around this by probing
the QSPI flash in the bootscript.

Other boards don't show this behaviour because they probe the flash
to fetch the bootloader environment settings.