で、emerge linux-firmwareするのですが、

The following license changes are necessary to proceed:
 (see "package.license" in the portage(5) man page for more details)
# required by =sys-kernel/linux-firmware-20200316 (argument)
=sys-kernel/linux-firmware-20200316 linux-fw-redistributable no-source-code


# echo  sys-kernel/linux-firmware linux-fw-redistributable no-source-code >> /etc/portage/package.license

これでemerge linux-firmwareを実行しましょう。

# emerge linux-firmware
Calculating dependencies... done!

- sys-firmware/radeon-ucode-20160628::gentoo (masked by: radeon-ucode license(s))
For more information, see the MASKED PACKAGES section in the emerge
man page or refer to the Gentoo Handbook.

>>> Verifying ebuild manifests
>>> Running pre-merge checks for sys-kernel/linux-firmware-20200316
 * Your boot partition is not mounted at /boot.
 * Please mount it and retry.
 * ERROR: sys-kernel/linux-firmware-20200316::gentoo failed (pretend phase):
 *   /boot not mounted
 * Call stack:
 *              , line 125:  Called pkg_pretend
 *   linux-firmware-20200316.ebuild, line  78:  Called mount-boot_pkg_pretend
 *                mount-boot.eclass, line  87:  Called mount-boot_check_status
 *                mount-boot.eclass, line  72:  Called die
 * The specific snippet of code:
 *              die "/boot not mounted"
 * If you need support, post the output of `emerge --info '=sys-kernel/linux-firmware-20200316::gentoo'`,
 * the complete build log and the output of `emerge -pqv '=sys-kernel/linux-firmware-20200316::gentoo'`.
 * The complete build log is located at '/var/tmp/portage/sys-kernel/linux-firmware-20200316/temp/build.log'.
 * The ebuild environment file is located at '/var/tmp/portage/sys-kernel/linux-firmware-20200316/temp/die.env'.
 * Working directory: '/var/tmp/portage/sys-kernel/linux-firmware-20200316/homedir'
 * S: '/var/tmp/portage/sys-kernel/linux-firmware-20200316/work/linux-firmware-20200316'

mount /bootを実行してからemergeしなおしました。