dfu: Fix build issue with gcc8
dfu: Fix build issue with gcc8

We get the following warning when buiding with gcc8:

	error: 'packed' attribute ignored for type 'u32_t *'
	{aka 'unsigned int *'} [-Werror=attributes]

Use UNALIGNED_GET() to access the data since we don't know the
alignment.

Signed-off-by: Kumar Gala <a5e5248af4cd4f0ed8c515f61d40a6e2db46a66e@linaro.org>

explora26/zephyr,ldts/zephyr,ldts/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,explora26/zephyr,nashif/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,ldts/zephyr,explora26/zephyr,ldts/zephyr,ldts/zephyr,explora26/zephyr,GiulianoFranchetto/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,nashif/zephyr,GiulianoFranchetto/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,Vudentz/zephyr,finikorg/zephyr,nashif/zephyr,nashif/zephyr,finikorg/zephyr,galak/zephyr,GiulianoFranchetto/zephyr,GiulianoFranchetto/zephyr,Vudentz/zephyr,Vudentz/zephyr,galak/zephyr,finikorg/zephyr,Vudentz/zephyr,GiulianoFranchetto/zephyr,galak/zephyr,explora26/zephyr,nashif/zephyr