gpt: allow spaces in partition list

gpt: allow spaces in partition list

This allows spliting partition list to several lines in environment file

ex:
--------------------
gpt_partition_list=
	name=boot1,size=5MiB,start=0x100000;
	name=boot2,size=5MiB;
	name=rootfs1,size=70MiB;
	name=rootfs2,size=70MiB;
	name=overlay1,size=20MiB;
	name=overlay2,size=20MiB;
	name=art,size=4MiB;

Signed-off-by: Mikhail Kshevetskiy <mikhail.kshevetskiy@iopsys.eu>
Reviewed-by: Sam Protsenko <semen.protsenko@linaro.org>
Tested-by: Sam Protsenko <semen.protsenko@linaro.org>
Reviewed-by: Simon Glass <sjg@chromium.org>