Replace buildroot prompt from '#' to $USER@buildroot:~#

Replace buildroot prompt from '#' to $USER@buildroot:~# by adding
/etc/profile file to rootfs overlay, this needs to be done as '#'
appears in kernel logs also which results into CI assuming the boot is
complete, it tries to run further comands and fails.

profile file is copied from buildroot code, and PS1 variable is modified
on top of default file available in buildroot.

Signed-off-by: Jagdish Gediya <jagdish.gediya@arm.com>