cpu: change udev buffer size from default value to 512 KB

cpu: change udev buffer size from default value to 512 KB

Between udev_monitor_enable_receiving() and the start of g_main_loop, a lot of udev events accumulate, and G_IO_ERR occurs in udev.

In vim3 target, many udev events occur when booting.
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KERNEL[3947.849435] add      /kernel/slab/:atA-0000192/cgroup/dentry(660:storaged.service) (cgroup)
KERNEL[3947.849544] add      /kernel/slab/inode_cache/cgroup/inode_cache(660:storaged.service) (cgroup)
KERNEL[3947.851730] add      /kernel/slab/sock_inode_cache/cgroup/sock_inode_cache(660:storaged.service) (cgroup)
KERNEL[3947.851822] add      /kernel/slab/:t-0000256/cgroup/kmalloc-256(660:storaged.service) (cgroup)
KERNEL[3947.851964] add      /kernel/slab/:t-0000512/cgroup/kmalloc-512(660:storaged.service) (cgroup)
KERNEL[3947.852916] add      /kernel/slab/:tA-0000192/cgroup/cred_jar(660:storaged.service) (cgroup)
KERNEL[3947.852971] add      /kernel/slab/:t-0000128/cgroup/kmalloc-128(660:storaged.service) (cgroup)
KERNEL[3947.853031] add      /kernel/slab/proc_inode_cache/cgroup/proc_inode_cache(660:storaged.service) (cgroup)
KERNEL[3947.853760] add      /kernel/slab/:tA-0000896/cgroup/mm_struct(660:storaged.service) (cgroup)
KERNEL[3947.853807] add      /kernel/slab/:tA-0000176/cgroup/vm_area_struct(660:storaged.service) (cgroup)
KERNEL[3947.853831] add      /kernel/slab/:tA-0000064/cgroup/anon_vma_chain(660:storaged.service) (cgroup)
KERNEL[3947.853848] add      /kernel/slab/anon_vma/cgroup/anon_vma(660:storaged.service) (cgroup)
==============================================================

Default value of udev buffer: /proc/sys/net/core/rmem_default (212992 byte on vim3 target)

Change-Id: I810a4f872b23e0f4b8e3ed8b68cdb5915ce1b0a3