tools/configure: drop BASH configure variable

tools/configure: drop BASH configure variable

This is a weird variable to have in the first place.  The only user of it is
XSM's CONFIG_SHELL, which opencodes a fallback to sh.  The scripts are shebang
sh, which is already necessary to support non-Linux build environments.

Make the mkflask.sh and mkaccess_vector.sh scripts executable, drop the
CONFIG_SHELL, and drop the $BASH variable to prevent further use.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>