um: Makefile: use bash from the environment

Set Makefile SHELL to bash instead of /bin/bash for better portability.

Some systems do not install binaries to /bin, and therefore do not
provide /bin/bash. This includes Linux distros which intentionally avoid
implementing the Filesystem Hierarchy Standard (FHS), such as NixOS and
Guix System.

The recipies inside arch/um/Makefile don't require top-level Bash to
build, and setting "SHELL" to "bash" makes Make pick the Bash executable
from the environment, hence this patch.

Changes since last roll:
- Rebase onto a more recent commit on the master branch.
- Remove a dangling in-text citation from the change log.
- Reword the change log.

Signed-off-by: Yueh-Shun Li <shamrocklee@posteo.net>
Reviewed-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: Richard Weinberger <richard@nod.at>