[build] Don't define special symbols in rodso.ld

This changes rodso.ld so its only effect is to produce the
desired optimal layout. It no longer defines special symbols for
use by scripts/gen-rodso-code.sh. (The rodso.ld layout is now what
will become the default for lld with -z rodynamic for these cases
once some corner case issues are resolved in lld.)

Instead, scripts/gen-rodso-code.sh is slightly reworked to glean
the values for the CODE_{START,END} and DATA_{START,END}_dynsym
constants from ELF program and section headers rather than from
symbol values defined by the linker script. userboot's generated
linker script for vDSO access uses linker-script arithmetic to
align the value from the linker's built-in _end symbol rather
than relying on a CODE_END symbol being defined by rodso.ld.

Change-Id: I200b687824c36ec3bd9e7d9d2a25464727ba09ee

[organization] move essential userspace components to system/core/...

userboot, devmgr, acpisvc, crashlogger, netsvc are essential to the
system booting and running. Pretty much everything else in system/uapp/..
is nonessential.

Change-Id: Ibbcf2b1c388bdefc28699226eab02e90aab895f0