[config] Allow user to specify custom CONFIG_SHELL

In case the shell the user wants to use as CONFIG_SHELL is located in a
weird place (eg. /opt/bash/bin/bash), or is weirdly named (eg. bash-4),
let the user enter the patch to the shell.

 # Compute powerpc-specific values

 . "${CT_LIB_DIR}/scripts/build/arch/powerpc-powerpc64.sh-common"