ipq40xx: convert IMAGE_SIZE/KERNEL_SIZE/BLOCKSIZE to kiB

ipq40xx: convert IMAGE_SIZE/KERNEL_SIZE/BLOCKSIZE to kiB

This cosmetical patch converts IMAGE_SIZE, KERNEL_SIZE and
BLOCKSIZE definitions to kilobytes, as this is consistent and
easier to read/type.

An exception was made for asus_rt-ac58u, where the IMAGE_SIZE of
20439364 cannot be divided by 1024 (and also does not seem to
match anything in DTS).

Build-tested for all devices.

Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>