Add missing growroot initscript and pkg-map entries for Gentoo.
growpart was added to Gentoo with [1]
Update the readme to reflect reality too (fedora added with
I5630dc638f85b1e80795826ef36a306632075460)
[1] https://packages.gentoo.org/packages/sys-fs/growpart
Closes-Bug: #1539273
Change-Id: I29056c7297489ec04f37757dbe33976901eceb49
