Bug#952906: [live-build] inefficiency in FIRMWARE_PACKAGES list construction

[jnqnfe@gmail.com]

Package: live-build
Version: 1:20191221
Owner: jnqnfe@gmail.com
Severity: minor

the logic used to construct FIRMWARE_PACKAGES suffers from an
inefficiency.

where multiple archive areas are used, the code on each per-archive-
area loop is:
 1) fetching the archive area contents file (compressed)
 2) **appending** the output of decompressing to a file
 3) searching the file for firmware references, adding to the list

this means that obviously there will be duplicates in the final list.
not that it means anything is broken, but it is flawed in terms of
efficiency.

patch to be submitted via salsa soon