init: Allow basic environment variable expansion in env assignment

Allow environment variable expansion in 'export' command if the
value begins with "${_var_}" (where "${_var_}" will be replaced
with the current value of environment variable _var_).  Expansion
will only occur one time.

e.g.
export BOOTCLASSPATH ${BOOTCLASSPATH}:/system/framework/abc.jar
(cherry picked from commit 7940299507f15055851a6e8b61ae8d8a7d7f3484)

Change-Id: If9e4adfa1aba2c7a4ca243978ced3c46c2cb4d73