mm: Improve build

Move modem related meson conf to mm directory
7 jobs for wip/sadiq/improve-build in 9 minutes and 16 seconds (queued for 2 seconds)
Status Name Job ID Coverage
  Build
passed build-pureos-gcc-byzantium #376056
librem5

00:01:52

 
  Test
passed check-po #376058
librem5

00:00:22

passed test:debian-gcc #376057
librem5

00:01:40

20.7%
 
  Package
passed package-pureos-byzantium #376059
librem5

00:02:16

passed package-pureos-byzantium:arm64 #376060
librem5:arm64

00:05:42

 
  Test Package
passed autopkgtest-pureos-byzantium-package #376061
librem5

00:01:24

passed lintian-pureos-byzantium-package #376062
librem5

00:00:50