Creating logical DOS partitions with fdisk (Fedora 27, fdisk from
util-linux 2.30.2) results in extended partition of type 0x05 being
created on the device. Partitioned like this, device is succesfully
recognized by Linux, but, due to algorithm in dos_partition(), not
Barebox.

Looking closer at the actual contents of MBR shows that while marked
as "Extended partition with CHS addressing" that partition still have
all of the LBA adressed fields filled correctly.

Given the above and the fact that similar code in Linux
kernel (block/partitions/msdos.c) does not make a distinction between
types 0x0f and 0x05, port Linux's is_extended_partition() and convert
the code to treat types 0x05, 0x0f and 0x85 the same way.

Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>