sys-firmware/intel-microcode: Add "minimal" USE flag

sys-firmware/intel-microcode: Add "minimal" USE flag

Due to previous change (commit eb9036f6f998c91c6bc021f73bc10ca1b5240ae7),
this package can become very large (or the resulting initramfs).

While the already introduced environment variable "MICROCODE_SIGNATURES" is
allowing you to set iucode_tool's "--scan-system" parameter to only
install ucode(s) supported by the currently available (=online) processor(s),
this doesn't work for binary package user(s).

The now added "minimal" USE flag (enabled by default) will set "--scan-system"
parameter for you. This will still allow you to select/blacklist ucode(s)
for all your hosts on your central build host using the "MICROCODE_SIGNATURES"
variable like before while giving each host the opportunity to only install
really supported ucode(s) which will reduces the file size of the resulting
initramfs.

Bug: https://bugs.gentoo.org/654638
Package-Manager: Portage-2.3.40, Repoman-2.3.9