tools: omapimage: fix corner-case in byteswap path

tools: omapimage: fix corner-case in byteswap path

Since commit 2614a208471e ("common: command: tempory buffer should
have size of command line buf"), there have been consistent Travis CI
failures on my builds (interestingly not for Tom, even though building
the same commit id) due to a SEGV in building the byteswapped
omapimage:
     	    arm: pcm051_rev3
     make[2]: *** [MLO.byteswap] Error 139
     	      	  		       ^^^ error code for a SEGV

Turns out that the word-based byte-swapping loop in omapimage.c is to
blame. With the loop condition
       while (swapped <= (sbuf->st_size / sizeof(uint32_t)))
there had been one-too-many iterations for all file sizes divisible by
the sizeof(uint32_t).  I.e. we had 1 iteration for 0 bytes (and also 1
through 3 bytes) and 2 iterations at 4 bytes... clearly overshooting
on 0 and 4 bytes.

This commit fixes the calculation of an up-rounded word-count and
makes sure to keep the zero-based loop-counter below the number of
words to be processed.

References: 2614a20 ("common: command: tempory buffer should have size of command line buf")
Fixes: 79b9ebb ("omapimage: Add support for byteswapped SPI images")
Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Reviewed-by: Martin Elshuber <martin.elshuber@theobroma-systems.com>