tools: imximage: Load a size that is multiple of 512

In order to mx53 ROM to properly load the U-boot image, its header size should
be multiple of 512 bytes.

This issue was observed with gcc 4.6.2/4.7.3, which caused data aborts:

U-Boot 2013.01-rc2-00172-gf8cfcf1-dirty (Dec 26 2012 - 13:13:28)

Board: MX53 LOCO
I2C:   ready
DRAM:  1 GiB
MMC:   FSL_SDHC: 0, FSL_SDHC: 1
In:    serial
Out:   serial
Err:   serial
CPU:   Freescale i.MX53 family rev2.1 at 1000 MHz
Reset cause: WDOG
Net:   FEC
Warning: FEC using MAC address from net device

Hit any key to stop autoboot:  0
data abort

    MAYBE you should read doc/README.arm-unaligned-accesses

pc : [<aff72220>]          lr : [<aff721fc>]
sp : af565e20  ip : af566918     fp : 00000000
r10: 00000003  r9 : affabb5b     r8 : af565f58
r7 : 00000000  r6 : 36747fff     r5 : af5668e8  r4 : 36747fff
r3 : af5668ec  r2 : af5668eb     r1 : 00000000  r0 : af5668e8
Flags: NzcV  IRQs off  FIQs off  Mode SVC_32
Resetting CPU ...

resetting ...

,and this patch fixes it.

Also, even though the ROUND macro is already defined in common.h,
the reason for redefining it in image.h is explained by Stefano Babic:

"I will remark a previous comment - even if including common.h seems a
good idea to avoid duplications, it makes tools like mkimage to depend
on the selected board, because <board>_config must run. Even if this is
not a problem for us u-boot developers, it becomes an issue when these
tools are included in distros (like u-boot-tools in Ubuntu) and cannot
be packaged."

Signed-off-by: Troy Kisky <troy.kisky@boundarydevices.com>
Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>