bump version name to 0.5.1 and code to 5100
make-release.build: fix timestamp generation to always output UTC time
Hide 'Image Size' from install screen when installing in /data

fixes #115 https://github.com/guardianproject/lildebi/issues/115
calculating free size of internal storage from '/data' and not '/data/debian'

When lildebi is installed first time on a device, then /data/debian folder is
not present. So when the install screen (of installing in internal storage)
tries to calculate the free size of internal storage from /data/debian then an
exception is generated as /data/debian is not present.  So instead of
calculating free size from /data/debian, we will now calculate it from /data

fixes #114 https://github.com/guardianproject/lildebi/issues/114
use canonical/absolute path when finding free bytes on install path

The SD Card aka "external storage" on Android can do weird stuff with the
path to support the multi-user stuff.  Using the canonical path seems to
help with some of the issues, and if that fails, then try the absolute path.
Debian's /etc/profile needs real `id -u`, so use busybox's

Android's `id` is in the PATH when this little snipped of code is run in
Debian.  So when /etc/profile is first executed and is setting the PATH,
it needs `id -u` to return only a numeric value.

fixes #88 https://github.com/guardianproject/lildebi/issues/88
fixes #116 https://github.com/guardianproject/lildebi/issues/116
always show "Open Terminal"; guide user to install it if its not already

Instead of hiding "Open Terminal" from the menu when Terminal Emulator is
not installed, instead guide the user to install it if they click that
menu item.
change menu name from "Preferences" to "Settings" following current standard
initialize "Install on Internal Storage" pref at startup

This was only ever inited when the pref changed, otherwise it used the
hard-coded default.  Now it will read the value from the stored prefs on
startup, and the default value is a standard part of that call.
add preference to limit image file to 4GB (that's the FAT file size limit)

The "External Storage" is often a FAT filesystem, but it turns out that it
is very difficult to reliably detect what filesystem the External Storage
is using.  So this is a workaround using a manual preference.

fixes #80 https://github.com/guardianproject/lildebi/issues/80
detect if Superuser is installed, if not, prompt user to install it

Lil' Debi currently only works with Superuser, and not SuperSU.  And it
requires an app to provide root access.  This moves the test to the first
screen, since it no longer attempts to get root access, but instead just
looks up the SU apps by packageName.

fixes #112 https://github.com/guardianproject/lildebi/issues/112