linux-raspi2: set two flavours to armhf kernel and change the flavour name to raspi4 for arm64 kernel

[Hui Wang]

[Impact]
It is risky to use single armhf kernel to support rpi2/3/4, since
upstream has a specific config for rpi2/3 and another config for rpi4,
the major difference are LPAE, HIGHMEM and VMSPLIT_3G, we already
met a bug that is intrduced by HIGHMEM and VMSPLIT_3G on rpi2/3,
so add two falvours for rpi2/3 and rpi4.
flavour 1: raspi2 (for rpi2/3, disable LPAE, HIGHMEM and VMSPLIT_3G)
flavour 2: raspi4 (for rpi4, enable LPAE, HIGHMEM and VMSPLIT_3G)

For arm64 kernel, we change the flavour name to raspi4, it supports
rpi2/3/4.

[Fix]
Change the related configuration files in the debian.raspi2.

[Test Case]
After apply the patches, and I built two flavours of armhf and
arm64 kernels, Then check the .config, they all meet the expectation.
And boot those kernels on the rpi2/3/4 boards. worked well.

[Regression Risk]
Low, just change the flavour name for arm64, and for armhf, the
flavour raspi4 is basically same as bofre (raspi2), the new flavour
raspi2 just disable LPAE/HIGHMEM/VMSPLIT_3G, it is same as the config
for rpi2/3 in the upstream.

For armhf kernel:
 flavour 1: raspi2 # support rpi2/3 boards
 flavour 2: raspi4 # support rpi4 boards (in theory support all rpi2/3/4 boards)

Form arm64 kernel:
 flavour 1: raspi4 # support rpi2/3/4 boards

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.