u-boot: Merge ostree's and systems uEnv.txt

u-boot: Merge ostree's and systems uEnv.txt

This is a proper fix for:
https://bugzilla.gnome.org/show_bug.cgi?id=755787

With this patch, an admin (system builder) can now:

1) Edit /usr/lib/ostree-boot/uEnv.txt
2) Deploy the new tree. OSTree will append system's uEnv.txt
   to the OSTree's managed uEnv.txt (loader/uEnv.txt).

It is common for u-boot systems to read in an extra env
from external /uEnv.txt. The same file OSTree uses to pass
in its env. With this patch /uEnv.txt now contains OSTree's
env + custom env added by system builders.

Closes: #466
Approved by: cgwalters