dev-environment: fix bitbake warning for shared build cache

When sharing Sstate directory between multiple builds BitBake
throws the following warning if BB_HASHSERVE_DB_DIR is not set [1].

$ bitbake obmc-phosphor-image

WARNING: Sstate directory is shared between several builds
(it is set via SSTATE_DIR to /home/javith/.yocto/sstate-cache),
but hash equivalency database is inside this particular build
directory /home/javith/openbmc/build/fvp

This prevents sstate reuse. It is recommended to set the database
location to a common path using BB_HASHSERVE_DB_DIR, for example:

BB_HASHSERVE_DB_DIR = "${SSTATE_DIR}"

Testing:
After configuring BB_HASHSERVE_DB_DIR = "${SSTATE_DIR}" in
build/conf/local.conf, the warning is no longer observed.

References:
[1]: https://docs.yoctoproject.org/bitbake/dev/bitbake-user-manual/bitbake-user-manual-ref-variables.html#term-BB_HASHSERVE_DB_DIR

Change-Id: I640a0052b4dfce5dac37b7ddd1fc6a40a37d8e21
Signed-off-by: Mohammed Javith Akthar M <mohammedjavitham@ami.com>