Use var ROOT_HOME to configure root home directory dynamically.

(From OE-Core rev: 341da5b6e46e0884d60e9462d1306d525fa99b94)

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