init: Fix bootchart trigger for emulator

When launched with "-bootchart <timeout>", the Android emulator appends
"androidboot.bootchart=<timeout>" to the kernel command line, which
signals /init to start bootcharting. However, the current implementation
of bootchart_init() in init/bootchart.cpp does not parse the timeout
value correctly, preventing bootcharting to be enabled on the emulator.

This bug was introduced by commit 841b263f ("Further refactoring of the
bootchart code"). Fix it to honor the "androidboot.bootchart" trigger.

Change-Id: I221fe2c2f40a3a04bd478c3a083f7723bc309c8c
Signed-off-by: Yu Ning <yu.ning@intel.com>