[scripts][build] Make scripts magenta-friendly

Allows scripts to run under dash in magenta.

Change-Id: I2913871ae92c8ca259fd00300e2171330782421e

[build] Add buildid to boot

On boot, magenta will now print a proper build id. If no git
information is found, it will show up as 'unknown'. Trees that contain
modifications will have '_DIRTY' as a suffix, and local builds will
additionally have '_LOCAL" at the end.

Change-Id: Iabb03d8cef0ff569e8d475cde2d291cc66a44ea2

[scripts] Search for bash with the env(1) trick.

This permits Magenta to build properly on systems where bash is not the default
shell and/or is installed in locations other than /bin.

Change-Id: Ic95e215c5d28b1a6e653bec53283540868c628f8

[magenta] Initial commit