`toolchain.sh` [already uses bash](https://github.com/pspdev/psptoolchain/blob/master/toolchain.sh#L1), and using it in the subscripts too allows us to [share functions](https://github.com/pspdev/psptoolchain/pull/42/files#diff-4aa568593ff3ba93745074d6762bc61aR6) with subshells using `export -f`, without having to do an extra `source ../common.sh` in each of the scripts.
The install script for psplibraries [specifically invokes bash](https://github.com/pspdev/psplibraries/blob/master/libraries.sh#L32) as well, so bash is pretty much a dependency already - unless you want to build the PSP SDK and for some reason don't want the libraries.

Also, every modern platform supplies bash in some form, either by default or as an optional library, so counting on its presence (even if it isn't the default) isn't an unreasonable assumption imo.

I can port it all (psptoolchain + psplibraries) away from bash if you really want me to, but it seems pretty unnecessary.