fetch2: Fix bug in file checksum generation

For a while its been puzzling me why connman-gnome rebuilds as often as it
does. It turns out you can trigger this with a new checkout of the metadata.

The SRC_URI that is causing the problems is:

SRC_URI = "file://images/*"

and rather oddly the results in checksums for a file "." being added to
the tree, e.g.:

('.', 'ab48a68186f0e0f277c21ef4cb390b4b')

The problem is that when iterating files lists, the checksum variable can
become set yet we don't break the out from the for loop, which leads to
odd (and non-deterministic) entries being added into the file checksum list.
The exact item added probably depends on the order of items on the disk.

Before this change, bitbake-diffsigs on connman-gnome:do_fetch would report:

This task depends on the checksums of files: [
('connman-signal-03.png', 'f6c16aee57b37b73793a2f1dea433ffa'),
('connman-signal-02.png', 'ad0cd22710c097d8174121fc1023c3be'),
('connman-signal-01.png', '8842bd83d2fa9ba56480df34c727c629'),
('null_check_for_ipv4_config.patch', 'a23271e41c9fe81551244d875106af10'),
('connman-signal-05.png', '808589e7e8d502b44c7b007e9e68d48c'),
('connman-signal-04.png', 'ab48a68186f0e0f277c21ef4cb390b4b'),
('null_check_for_ipv4_config.patch', 'a23271e41c9fe81551244d875106af10'),
('0001-Removed-icon-from-connman-gnome-about-applet.patch', 'e2d8269357c1e8c84291606da24eea85'),
('0001-Removed-icon-from-connman-gnome-about-applet.patch', 'e2d8269357c1e8c84291606da24eea85'),
('.', 'ab48a68186f0e0f277c21ef4cb390b4b')]

Afterwards:

This task depends on the checksums of files: [
('connman-signal-03.png', 'f6c16aee57b37b73793a2f1dea433ffa'),
('connman-signal-02.png', 'ad0cd22710c097d8174121fc1023c3be'),
('connman-signal-01.png', '8842bd83d2fa9ba56480df34c727c629'),
('null_check_for_ipv4_config.patch', 'a23271e41c9fe81551244d875106af10'),
('connman-signal-05.png', '808589e7e8d502b44c7b007e9e68d48c'),
('connman-signal-04.png', 'ab48a68186f0e0f277c21ef4cb390b4b'),
('null_check_for_ipv4_config.patch', 'a23271e41c9fe81551244d875106af10'),
('0001-Removed-icon-from-connman-gnome-about-applet.patch', 'e2d8269357c1e8c84291606da24eea85'),
('0001-Removed-icon-from-connman-gnome-about-applet.patch', 'e2d8269357c1e8c84291606da24eea85')]

which is correct and deterministic without the "." entry.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>