With several nice commits landing into the mainline of Linux Kernel for version 4.5, is it already possible to compile and boot a mainline kernel (e.g. 4.5-rc6) on a Raspberry Pi 2? If yes, then what is the simplest way to do so? What kernel configuration options do I need to set for basic Pi 2 hardware support (SD card, ethernet, usb, text mode video output)? Do I need some extra binary blobs?
While the Raspberry Pi 2 support is present in 4.5, you're also looking for a .config that will work. The changes for multi_v7_defconfig weren't ready for 4.5, so they're currently in linux-next. That defconfig is still missing a couple of things:
CONFIG_BCM2835_WDT=y (enables reboot/shutdown)
CONFIG_SND_BCM2835_SOC_I2S=y (support for external audio DACs, not the 3.5mm jack)
CONFIG_DMA_BCM2835=y (only used for i2s currently)
Even with all of this, graphics doesn't work without this series.
Note that if you're doing your own kernels, you want serial console so you can debug. Otherwise you'll be staring at a dead computer and wondering why things don't work.