abuild will not consider package up2date unless apk cache is up2date

with support for dynamic dependencies

use USBIMG_FREE to set how many kB you want to have as free space on
usb image.

The sda1 might be a sata or scsi harddisk so usb image might be sdb1 or sdc1.
The /dev/usbdisk link will always point to the first usb disk.

The APK_FILES does not contain the dependencies for the wanted packages
so we cannot use this variable when generating the index.

Basicly, let apk-tools deal with the dependencies rather than let abuild do it

This is so you can build the iso image without having the aports tree.

fixes #28

The tmprepo is a temporary repository used by abuild to handle the
dependencies of the newly built packages.

This will later be used to figure out the dynamic dependencies.

Run all subpackage functions and prepare the dirs first and create the
package itself later on. This will make it possible to automatically trace
the dependencies.

Also, speed up the detection of which makedeps are installed and which needs
to be built/installed. this requires apk-tools_pre11.

where <pkg> is package/subpackage name

fixes #23

sometimes it is possible to manually fix the problem, for example mount
the alpine device manually after modprobing the correct modules. Then it
makes sense to continue boot rather than just die.

set default packages dir to ~/packages