facilitate start up without bluetooth controller

facilitate start up without bluetooth controller

Add prefix '-' in the bluetooth service ExecStartPre
directive, to allow for errors: hciconfig will
return an error if there's no controller in the
system.

Bug-AGL: SPEC-3301
Signed-off-by: Raquel Medina <raquel.medina@konsulko.com>
Change-Id: Id4d5de549e73fe8f744681b33debd6043e1bc9ba
Reviewed-on: https://gerrit.automotivelinux.org/gerrit/c/AGL/meta-agl/+/24952
Tested-by: Jenkins Job builder account <agl-jobbuilder@automotivelinux.org>
ci-image-build: Jenkins Job builder account <agl-jobbuilder@automotivelinux.org>
ci-image-boot-test: Jenkins Job builder account <agl-jobbuilder@automotivelinux.org>
Reviewed-by: Scott Murray <scott.murray@konsulko.com>