Add CROSS_COMPILE option

In order to support multiple platform architectures such as ARM, this
patch enable you to cross-compile easily.

For example, when executing as below, you can build the binary files for
ARM architecture.
$ make CROSS_COMPILE="arm-linux-gnueabi-"

If you just execute 'make' without CROSS_COMPILE option, then the result
is the same as before.

Signed-off-by: Sangjung Woo <sangjung.woo@samsung.com>
Makefile: Clean systemd unit files

The below files are generated by 'make' command. However, these files
are not removed after 'make clean' command.
 - bootchart-done.service
 - bootchart-done.timer
 - bootchart.service

Signed-off-by: Sangjung Woo <sangjung.woo@samsung.com>
Makefile: Fix @EARLY_PREFIX@ error

After executing 'make' command, systemd's service files are generated
from their *.in files. However, "@EARLY_PREFIX@" string in *.in files is
not converted properly. As a result, service files generated have a raw
"@EARLY_PREFIX@".

This patch fixes this error.

Signed-off-by: Sangjung Woo <sangjung.woo@samsung.com>
Merge pull request #45 from again4you/cross_compile

Enabling the cross compile and  fixing some bugs in Makefile