Move sourced/shared scripts to boot-*-common
Rearrange common definitions in each boot script to make them more
similar
Add common support for %ARCH% substitutions in DI_WWW_HOME
Fix boot-alpha to use which_deb when looking for aboot
Merge mips* together, using boot-mips-common
we've already got code to copy the d-i images into a handy place for
users. Move it into a common function and reference that for all the
arches affected.