Fix TA panic in EPS generation

[akorb]

With the current state of the code, the fTPM TA panics with the following output if -D USE_PLATFORM_EPS is specified:

I/TA: Size=64
I/TA: EPS=128
E/TC:? 0 
E/TC:? 0 TA panicked with code 0x0
E/LD:  Status of TA bc50d971-d4c9-42c4-82cb-343fb7f37896
E/LD:   arch: arm
E/LD:  region  0: va 0x40004000 pa 0x06200000 size 0x002000 flags rw-s (ldelf)
E/LD:  region  1: va 0x40006000 pa 0x06202000 size 0x008000 flags r-xs (ldelf)
E/LD:  region  2: va 0x4000e000 pa 0x0620a000 size 0x001000 flags rw-s (ldelf)
E/LD:  region  3: va 0x4000f000 pa 0x0620b000 size 0x004000 flags rw-s (ldelf)
E/LD:  region  4: va 0x40013000 pa 0x0620f000 size 0x001000 flags r--s
E/LD:  region  5: va 0x40014000 pa 0x06275000 size 0x011000 flags rw-s (stack)
E/LD:  region  6: va 0x40078000 pa 0x00001000 size 0x046000 flags r-xs [0]
E/LD:  region  7: va 0x400be000 pa 0x00047000 size 0x01f000 flags rw-s [0]
E/LD:   [0] bc50d971-d4c9-42c4-82cb-343fb7f37896 @ 0x40078000
E/LD:  Call stack:
E/LD:   0x400aa374
E/LD:   0x400a988b
E/LD:   0x4007c0c1
E/LD:   0x40092be3
E/LD:   0x400a0e61

This happens because in _plat__GetEPS the EPSLen variable which is passed to TEE_GetPropertyAsBinaryBlock is not initialized. However, TEE_GetPropertyAsBinaryBlock expects this variable to be set to the size of the provided buffer, as explained here.

It also fixes that the getting the value of the property always failed, since the property is not registered specifically for the fTPM TA, but for the whole TEE here, i.e., use TEE_PROPSET_TEE_IMPLEMENTATION instead of TEE_PROPSET_CURRENT_TA.

In addition, it fixes that we might write out of buffer of the variable EndorsementSeed by giving its actual length to _plat__GetEntropy instead of the fixed EPS size (TEE_EPS_SIZE).

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