The kernel does not autodetect raid arrays as part of its boot process.

resq:/# uname -a
Linux resq 3.18.3 #1 SMP Tue Jan 20 11:49:36 EET 2015 x86_64 GNU/Linux

resq:/# gunzip -dc /proc/config.gz|grep -i _md_
CONFIG_MD_AUTODETECT=y
# CONFIG_MD_LINEAR is not set
# CONFIG_MD_RAID0 is not set
CONFIG_MD_RAID1=y
# CONFIG_MD_RAID10 is not set
# CONFIG_MD_RAID456 is not set
# CONFIG_MD_MULTIPATH is not set
# CONFIG_MD_FAULTY is not set

resq:/# dmesg|grep -i raid
[    0.575628] md: raid1 personality registered for level 1

resq:/# cat /proc/mdstat
Personalities : [raid1]
unused devices: <none>

resq:/# sfdisk -d /dev/sd[ab]
# partition table of /dev/sda
unit: sectors

/dev/sda1 : start=       64, size=  2056256, Id= b
/dev/sda2 : start=  2056320, size=268349760, Id=fd
/dev/sda3 : start=270406080, size=268349760, Id=fd
/dev/sda4 : start=538755840, size=438017328, Id=fd
# partition table of /dev/sdb
unit: sectors

/dev/sdb1 : start=       64, size=  2056256, Id= b
/dev/sdb2 : start=  2056320, size=268349760, Id=fd
/dev/sdb3 : start=270406080, size=268349760, Id=fd
/dev/sdb4 : start=538755840, size=438017328, Id=fd

resq:/# cat /proc/cmdline
root=/dev/ram rw vga=0 ipv6.disable=1 panic=30

Hands everything works fine:

resq:/# mdadm -A /dev/md0 /dev/sd[ab]2
mdadm: /dev/md0 has been started with 2 drives.

resq:/# mdadm -A /dev/md1 /dev/sd[ab]3
mdadm: /dev/md1 has been started with 2 drives.

resq:/# mdadm -A /dev/md2 /dev/sd[ab]4
mdadm: /dev/md2 has been started with 2 drives.

resq:/# cat /proc/mdstat
Personalities : [raid1]
md2 : active raid1 sda4[0] sdb4[1]
      219008576 blocks [2/2] [UU]

md1 : active raid1 sda3[0] sdb3[1]
      134174784 blocks [2/2] [UU]

md0 : active raid1 sda2[0] sdb2[1]
      134174784 blocks [2/2] [UU]

unused devices: <none>
resq:/# mdadm -D /dev/md0
/dev/md0:
        Version : 0.90
  Creation Time : Tue Jan 20 09:24:35 2015
     Raid Level : raid1
     Array Size : 134174784 (127.96 GiB 137.39 GB)
  Used Dev Size : 134174784 (127.96 GiB 137.39 GB)
   Raid Devices : 2
  Total Devices : 2
Preferred Minor : 0
    Persistence : Superblock is persistent

    Update Time : Tue Jan 20 09:30:48 2015
          State : clean
 Active Devices : 2
Working Devices : 2
 Failed Devices : 0
  Spare Devices : 0

           UUID : 202ffd29:ee258f3d:d6159ac9:ae4e48bd (local to host resq)
         Events : 0.1

    Number   Major   Minor   RaidDevice State
       0       8        2        0      active sync   /dev/sda2
       1       8       18        1      active sync   /dev/sdb2

resq:/# mdadm -D /dev/md[012]|grep -i version
        Version : 0.90
        Version : 0.90
        Version : 0.90

Additional information:
http://www.tts.lt/resq_config.gz
http://www.tts.lt/resq_dmesg.gz