bootchart: fix bootchart can not be triggered problem

bootchart uses a file on the data partition to decide if it should collect
data for bootchart, but the data partition will be mounted by the mount_all
command in the "on fs" section, and it will be only added into the action
queue when command "trigger fs" is executed, but that's after the
bootchart_init action (late_init).

This change makes bootchart_init a builtin command of init,
and make it executed as the first command of "on post-fs" section
which will be triggered after the "on fs" section.

This change also refactors the bootchart code to all be in bootchart.cpp.

Change-Id: Ia74aa34ca5b785f51fcffdd383075a549b2a99d9
Signed-off-by: Yongqin Liu <yongqin.liu@linaro.org>