Add env variable support in vm from the yaml

[cozkul]

When I try to run curl using app/elfloader/scripts/run/qemu-x86_64-9pfs-curl.sh curl complains the user home directory cannot be found. Issue can be resolved using make menuconfig > Library Configuration > posix-environ > Compiled-in Environment Variables > Add (HOME=/)

It could be easier if the user had a .env file specified in the YAML file. This could also make building of databases easier, as they also need env vars such as DB_USER, DB_PASS, etc.