boot-time-monitor: make data directory configurable

Boot time monitor is storing all boot time data as csv format inside
/usr/share/boot-time-monitor. Under this commit making this
configurable so that platforms can pass that value based on requirement.

Google-Bug-Id: 326579702
Change-Id: I4e7f8b39b7beb12af8ee1f7ac7d4820e37d7c218
Signed-off-by: Munawar Hussain <munawarhussain@google.com>