eglibc: force make to use /bin/bash

eglibc: force make to use /bin/bash

The eglibc core build still has a number of issues with /bin/dash.
Recently found is both sysdeps/unix/make-syscalls.sh and it's output,
which make calls via SHELL do not play well with /bin/dash. By force
make to use /bin/bash via SHELL setting it works correctly.

Currenly known issues are: make-syscalls.sh line with a bad substitution,
which can be corrected by:
     vdso_symver="$(printf '%s\n' ${vdso_symver} | sed 's/\./_/')"

Following that there is an issue with emiting a '\n' through a second
echo and then to the compiler. There maybe more issues beyond that.

[YOCTO #3080]

(From OE-Core rev: 9d002f7cdc5309c4d850a76e4fd73ff04c980a07)

Signed-off-by: Saul Wold <sgw@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>