Hang-up scenario before this patch:
At the time `udelay()` is called, the current value in
    the SYSTICK timer (`systick->val`) may be very close to, but greater
    than the number of clocks left to tick (`clc`).
Since `systick->val` is greater than `clc`, we use the `while` loop from
    the `else` block. But this `while` loops only checks that `systick->val`
    is greater than `(tmp - clc)` which is a very small positive integer
    (because `tmp` is very close to `clc`.) The `while` loop will now
    terminate only if we manage to catch `systick->val` in a very thin
    range between 0 and `(tmp - clc)`. The comparison inside the `while`
    loop condition takes a considerable number of CPU clocks, that makes it
    hard to find `systick->val` between 0 and `(tmp - clc)`, we just step
    over this small range.

This bug manifested when erasing NOR flash on EA-LPC1788-32: the
`udelay(1)` call in `flash_status_check()` in `drivers/mtd/cfi_flash.c`
was hanging.