scripts: remove hardcoding of bash hashbang

boards/arduino-mega2560/dist/debug.sh

boards/arduino-mega2560/dist/debug_srv.sh

boards/msba2-common/tools/flashutil.sh

boards/pttu/tools/openocd-pttu.sh

boards/x86-multiboot-common/dist/link

dist/tools/drone-scripts/build_and_test.sh

dist/tools/eclipsesym/cmdline2xml.sh

dist/tools/pr_check/check_labels.sh

dist/tools/pr_check/pr_check.sh

dist/tools/testsuite/build_and_generate_html.sh

dist/tools/testsuite/generate_html.sh

dist/tools/testsuite/parse_buildlog.sh

dist/tools/testsuite/run_tests.sh

dist/tools/toolchains/build_gnuarm.sh

dist/tools/toolchains/build_gnuarm_naked.sh

dist/tools/toolchains/build_x86.sh

dist/tools/travis-scripts/build_and_test.sh

dist/tools/usb-serial/find-tty.sh

pkg/libfixmath/checkout.sh

pkg/libfixmath/perform-svn-checkout.sh