Avoid building all software packages for simple SoCs

[enjoy-digital]

 "liballoc",
 "libm",
 "libdyld",
 "libunwind"

are not needed for simple SoCs and libdyld is only implemented for or1k. (ie we are not longer able to build lm32 SoCs).

Can we only keep this for misoc_software_packages:

misoc_software_packages = [
    "libcompiler_rt",
    "libbase",
    "libnet",
    "bios"
]

and add the others packages with add_software_package in ARTIQ's targets?

[enjoy-digital]

thanks!