ipq40xx: re-enable EA6350v3, EA8300, MR8300 builds

ipq40xx: re-enable EA6350v3, EA8300, MR8300 builds

Raise the kernel size from 3 MB to 5 MB for EA6350v3, EA8300 and MR8300,
and correspondingly reduce the rootfs size by 2 MB:
  * modify partition definitions in related .dts files
  * modify device kernel/image sizes in generic.mk

Update to compat-version 2.0 to force factory image usage on sysupgrade,
noting the current version 1.1 is an unreleased update for DSA migration.

Also update the compat-version message, explaining the need to run one of
the following console commands to update U-Boot's kernel-size variable
before flashing the OpenWrt factory image.

    fw_setenv kernsize 500000          # (OpenWrt command line)

    setenv kernsize 500000 ; saveenv   # (U-Boot serial console)

Finally, re-enable the 3 devices.

Reviewed-by: Robert Marko <robimarko@gmail.com>
Tested-by: Nicolas TORMO <badulesia.granieri@gmail.com> # MR8300
Tested-by: Tony Ambardar <itugrok@yahoo.com> # EA6350v3
Signed-off-by: Tony Ambardar <itugrok@yahoo.com>