Enable UUID mount support in berryboot.img busybox

[macmpi]

Makes it consistent with busybox-static to similarly allow boot partition mount by UUID if need be, as available from bootdev statement in cmdline.txt.
Particularly useful if bbloader.img is not used (hence /boot not mounted) and berryboot-init needs to mount /boot at some point; since cmdline.txt may refer UUID, we just need to be able to use it.
...and anyhow consistency is good :)

Note: this busybox.config still comes from legacy 0.28.x, although actual distribution used is 0.29.x: one would need to line them-up too.

[maxnet]

Doesn't already work?
There is code somewhere to resolve the reference using libblkid.

[macmpi]

Did not have a chance to check yet, but anyway thought would make sense to just have same for consistency.

[macmpi]

Here are my tests mounting from berryboot console:

# mount -U 0720-C5A4 /toto
mount: invalid option -- U

# mount 0720-C5A4 /toto
mount: mounting 0720-C5A4 on /toto failed: No such file or directory

# mount UUID="0720-C5A4" /toto
mount: mounting UUID=0720-C5A4 on /toto failed: No such file or directory

# mount UUID=0720-C5A4 /toto
mount: mounting UUID=0720-C5A4 on /toto failed: No such file or directory

# mount --uuid 0720-C5A4 /toto
mount: mounting 0720-C5A4 on /toto failed: No such file or directory

# mount /dev/disk/by-uuid/0720-C5A4 /toto

Only the last (cumbersome) one worked.
According to this busybox patch:

Summary:                    PATH: Mounting by volume UUID and label
Description: 
This patch adds support of -L and -U mount switches and also specification
of device in command line in one of the form UUID=... and LABEL=...
Requires libuuid and libblkid.

So I assume setting the option for that berryboot.img busybox is desirable.
Thanks.

[macmpi]

To further clarify: the point of this PR is to allow easy & consistent mount by UUID from either console (helps debugging) or berryboot-init scripts.
Indeed mount UUID=XXX /mntpoint is the expected syntax, now bbloader also supports that convention using cmdline.txt parameter.

(obviously berryboot2 app internally figures-out how to mount by UUID without this, leveraging libblkid.... but not obvious at console or berryboot-init scripts levels)

Thanks for consideration.