support: properly check bash is available

The current check is broken, as it only checks if the user's login
shell is bash, not what the system shell is.

Mimick the sequence found in the top-level Makefile to search for
bash, except for the fallback case, where we explicitly check that
'sh' is bash, by checking if it sets $BASH, so we know the fallback
case, in the top-level Makefile, to use 'sh' will indeed use bash.

Remove superfluous semi-colons ';' at the end of lines, they are
not needed in a shell script (this is not C!)

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Peter Korsgaard <jacmet@uclibc.org>
Cc: Andrew Barnes <andy@outsideglobe.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>