When booting with --bootchart, OSv will print a summary of where is our boot
time being spent up to the point right before our execution of main.

This mechanism can be extended later to keep measuring it later using other
facilities to account for the application, etc.

Example output:

OSv v0.05-156-gd3918a1
    disk read (real mode): 132.94ms, (+132.94ms)
    .init functions: 146.10ms, (+13.16ms)
    SMP launched: 147.57ms, (+1.47ms)
    RCU initialized: 150.61ms, (+3.04ms)
    VFS initialized: 154.08ms, (+3.46ms)
    Network initialized: 160.79ms, (+6.71ms)
    pvpanic done: 162.31ms, (+1.52ms)
    pci enumerated: 171.45ms, (+9.14ms)
    drivers probe: 171.46ms, (+0.02ms)
    drivers loaded: 182.52ms, (+11.06ms)
    ZFS mounted: 2116.32ms, (+1933.80ms)
    Total time: 2116.70ms, (+0.38ms)

Signed-off-by: Glauber Costa <glommer@cloudius-systems.com>
Reviewed-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>