fastboot: Check if partition really exist in getvar_has_slot()

fastboot: Check if partition really exist in getvar_has_slot()

Currently getvar_has_slot() invocation for "boot" and "system"
partitions always returns affirmative response regardless the fact of
existence of these partitions, which leads to impossibility to flash them
on old non-A/B AOSP setups, where _a/_b suffixes aren't used:

$ fastboot flash boot boot.img
Sending 'boot__a' (11301 KB)    OKAY [  0.451s]
Writing 'boot__a'               FAILED (remote: 'cannot find partition')
fastboot: error: Command failed

Although partition layout is:
-> part list mmc 0
Partition Map for MMC device 0  --   Partition Type: EFI

Part	Start LBA	End LBA		Name
	Attributes
	Type GUID
	Partition GUID
  1	0x00000800	0x000107ff	"boot"
	attrs:	0x0000000000000000
	type:	ebd0a0a2-b9e5-4433-87c0-68b6b72699c7
	guid:	ea2e2470-db4a-d646-b828-10167f736d63
  2	0x00010800	0x000127ff	"environment"
	attrs:	0x0000000000000000
	type:	ebd0a0a2-b9e5-4433-87c0-68b6b72699c7
	guid:	10a819d2-6004-3d48-bd87-114e2a796db9
  3	0x00012800	0x0001a7ff	"recovery"
	attrs:	0x0000000000000000
	type:	ebd0a0a2-b9e5-4433-87c0-68b6b72699c7
	guid:	9ea116e4-8a34-0c48-8cf5-2fe9480f56cd
  4	0x0001a800	0x0031a7ff	"system"
	attrs:	0x0000000000000000
......

This patch adds checks of existence for requested partitions
on eMMC/NAND.

Fixes: f73a7df984a9 ("net: fastboot: Merge AOSP UDP fastboot")
Signed-off-by: Igor Opaniuk <igor.opaniuk@toradex.com>
Signed-off-by: Sam Protsenko <semen.protsenko@linaro.org>