build: from_file returns option
build: from_file returns option

	- BuildFile::from_file returns `Option<BuildFile>`.
	- Change `get_host_arch` to only get the architecture and
	  `assign_host_arch` to set it in `BuildFile`

Signed-off-by: Alberto Corona <0c11d463c749db5838e2c0e489bf869d531e5403@albertocorona.com>

0X1A/mpm