mediatek: mt7623: use bash for generating bootable images

mediatek: mt7623: use bash for generating bootable images

It turns out that 'echo -e' isn't portable; it doesn't work in the dash
builtin echo and Ubuntu users are complaining.

I can't even get octal (specified by POSIX) to work consistently because
those  variants of 'echo' which *do* support -e don't seem to interpret
octalwithout it.

I could switch to /bin/echo but using -e with that isn't actually
portable *either* even though it works today.

For now just stick with bash, and use its builtin. We may end up using
something else entirely; perhaps perl.

Signed-off-by: David Woodhouse <dwmw2@infradead.org>