Use Gentoo.EMERGE_UPDATE_OPTS for invoke_emerge call to install pkgs_boot
Using Gentoo.EMERGE_UPDATE_OPTS allows for greater maintainability than directly
specifying `--update` and `--deep`.
The configured `opts` for `invoke_emerge` enable the package manager to replace
blocking packages. This is required to successfully replace
`sys-kernel/installkernel-gentoo` (which is already installed in the gentoo
stage3 tar ball) with the requested `sys-kernel/installkernel-systemd-boot`.